description: <image>
  
  William has a favorite bracket sequence. Since his favorite sequence is quite big he provided it to you as a sequence of positive integers c_1, c_2, ..., c_n where c_i is the number of consecutive brackets "(" if i is an odd number or the number of consecutive brackets ")" if i is an even number.
  
  For example for a bracket sequence "((())()))" a corresponding sequence of numbers is [3, 2, 1, 3].
  
  You need to find the total number of continuous subsequences (subsegments) [l, r] (l ≤ r) of the original bracket sequence, which are regular bracket sequences.
  
  A bracket sequence is called regular if it is possible to obtain correct arithmetic expression by inserting characters "+" and "1" into this sequence. For example, sequences "(())()", "()" and "(()(()))" are regular, while ")(", "(()" and "(()))(" are not.
  
  Input
  
  The first line contains a single integer n (1 ≤ n ≤ 1000), the size of the compressed sequence.
  
  The second line contains a sequence of integers c_1, c_2, ..., c_n (1 ≤ c_i ≤ 10^9), the compressed sequence.
  
  Output
  
  Output a single integer — the total number of subsegments of the original bracket sequence, which are regular bracket sequences.
  
  It can be proved that the answer fits in the signed 64-bit integer data type.
  
  Examples
  
  Input
  
  
  5
  4 1 2 3 1
  
  
  Output
  
  
  5
  
  
  Input
  
  
  6
  1 3 2 1 2 4
  
  
  Output
  
  
  6
  
  
  Input
  
  
  6
  1 1 1 1 2 2
  
  
  Output
  
  
  7
  
  Note
  
  In the first example a sequence (((()(()))( is described. This bracket sequence contains 5 subsegments which form regular bracket sequences:
  
    1. Subsequence from the 3rd to 10th character: (()(()))
    2. Subsequence from the 4th to 5th character: ()
    3. Subsequence from the 4th to 9th character: ()(())
    4. Subsequence from the 6th to 9th character: (())
    5. Subsequence from the 7th to 8th character: ()
  
  
  
  In the second example a sequence ()))(()(()))) is described.
  
  In the third example a sequence ()()(()) is described.
solution: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.io.PrintStream;
  import java.io.IOException;
  import java.lang.reflect.Field;
  import java.nio.charset.StandardCharsets;
  import java.io.UncheckedIOException;
  import java.io.Closeable;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   */
  public class Main {
      public static void main(String[] args) throws Exception {
          Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 29);
          thread.start();
          thread.join();
      }
  
      static class TaskAdapter implements Runnable {
          @Override
          public void run() {
              InputStream inputStream = System.in;
              OutputStream outputStream = System.out;
              FastInput in = new FastInput(inputStream);
              FastOutput out = new FastOutput(outputStream);
              CCompressedBracketSequence solver = new CCompressedBracketSequence();
              solver.solve(1, in, out);
              out.close();
          }
      }
  
      static class CCompressedBracketSequence {
          Debug debug = new Debug(false);
  
          public void solve(int testNumber, FastInput in, FastOutput out) {
              int n = in.ri();
              long[] c = in.rl(n);
              long ans = 0;
              for (int i = 0; i < n; i++) {
                  if (i % 2 == 1) {
                      continue;
                  }
                  long l = 1;
                  long r = c[i];
                  long local = 0;
                  for (int j = i + 1; j < n && r >= 0; j++) {
                      if (j % 2 == 0) {
                          l += c[j];
                          r += c[j];
                      } else {
                          l -= c[j];
                          r -= c[j];
                          if (l <= 0) {
                              if (r >= 0) {
                                  local += -l + 1;
                              } else {
                                  local += r - l + 1;
                              }
                              l = 0;
                          }
                      }
                  }
                  debug.debug("i", i);
                  debug.debug("local", local);
                  ans += local;
              }
  
              out.println(ans);
          }
  
      }
  
      static class FastOutput implements AutoCloseable, Closeable, Appendable {
          private static final int THRESHOLD = 32 << 10;
          private OutputStream writer;
          private StringBuilder cache = new StringBuilder(THRESHOLD * 2);
          private static Field stringBuilderValueField;
          private char[] charBuf = new char[THRESHOLD * 2];
          private byte[] byteBuf = new byte[THRESHOLD * 2];
  
          static {
              try {
                  stringBuilderValueField = StringBuilder.class.getSuperclass().getDeclaredField("value");
                  stringBuilderValueField.setAccessible(true);
              } catch (Exception e) {
                  stringBuilderValueField = null;
              }
              stringBuilderValueField = null;
          }
  
          public FastOutput append(CharSequence csq) {
              cache.append(csq);
              return this;
          }
  
          public FastOutput append(CharSequence csq, int start, int end) {
              cache.append(csq, start, end);
              return this;
          }
  
          private void afterWrite() {
              if (cache.length() < THRESHOLD) {
                  return;
              }
              flush();
          }
  
          public FastOutput(OutputStream writer) {
              this.writer = writer;
          }
  
          public FastOutput append(char c) {
              cache.append(c);
              afterWrite();
              return this;
          }
  
          public FastOutput append(long c) {
              cache.append(c);
              afterWrite();
              return this;
          }
  
          public FastOutput println(long c) {
              return append(c).println();
          }
  
          public FastOutput println() {
              return append('\n');
          }
  
          public FastOutput flush() {
              try {
                  if (stringBuilderValueField != null) {
                      try {
                          byte[] value = (byte[]) stringBuilderValueField.get(cache);
                          writer.write(value, 0, cache.length());
                      } catch (Exception e) {
                          stringBuilderValueField = null;
                      }
                  }
                  if (stringBuilderValueField == null) {
                      int n = cache.length();
                      if (n > byteBuf.length) {
                          //slow
                          writer.write(cache.toString().getBytes(StandardCharsets.UTF_8));
  //                writer.append(cache);
                      } else {
                          cache.getChars(0, n, charBuf, 0);
                          for (int i = 0; i < n; i++) {
                              byteBuf[i] = (byte) charBuf[i];
                          }
                          writer.write(byteBuf, 0, n);
                      }
                  }
                  writer.flush();
                  cache.setLength(0);
              } catch (IOException e) {
                  throw new UncheckedIOException(e);
              }
              return this;
          }
  
          public void close() {
              flush();
              try {
                  writer.close();
              } catch (IOException e) {
                  throw new UncheckedIOException(e);
              }
          }
  
          public String toString() {
              return cache.toString();
          }
  
      }
  
      static class Debug {
          private boolean offline;
          private PrintStream out = System.err;
  
          public Debug(boolean enable) {
              offline = enable && System.getSecurityManager() == null;
          }
  
          public Debug debug(String name, int x) {
              if (offline) {
                  debug(name, "" + x);
              }
              return this;
          }
  
          public Debug debug(String name, long x) {
              if (offline) {
                  debug(name, "" + x);
              }
              return this;
          }
  
          public Debug debug(String name, String x) {
              if (offline) {
                  out.printf("%s=%s", name, x);
                  out.println();
              }
              return this;
          }
  
      }
  
      static class FastInput {
          private final InputStream is;
          private byte[] buf = new byte[1 << 13];
          private int bufLen;
          private int bufOffset;
          private int next;
  
          public FastInput(InputStream is) {
              this.is = is;
          }
  
          public void populate(long[] data) {
              for (int i = 0; i < data.length; i++) {
                  data[i] = readLong();
              }
          }
  
          private int read() {
              while (bufLen == bufOffset) {
                  bufOffset = 0;
                  try {
                      bufLen = is.read(buf);
                  } catch (IOException e) {
                      bufLen = -1;
                  }
                  if (bufLen == -1) {
                      return -1;
                  }
              }
              return buf[bufOffset++];
          }
  
          public void skipBlank() {
              while (next >= 0 && next <= 32) {
                  next = read();
              }
          }
  
          public int ri() {
              return readInt();
          }
  
          public long[] rl(int n) {
              long[] ans = new long[n];
              populate(ans);
              return ans;
          }
  
          public int readInt() {
              boolean rev = false;
  
              skipBlank();
              if (next == '+' || next == '-') {
                  rev = next == '-';
                  next = read();
              }
  
              int val = 0;
              while (next >= '0' && next <= '9') {
                  val = val * 10 - next + '0';
                  next = read();
              }
  
              return rev ? val : -val;
          }
  
          public long readLong() {
              boolean rev = false;
  
              skipBlank();
              if (next == '+' || next == '-') {
                  rev = next == '-';
                  next = read();
              }
  
              long val = 0;
              while (next >= '0' && next <= '9') {
                  val = val * 10 - next + '0';
                  next = read();
              }
  
              return rev ? val : -val;
          }
  
      }
  }
  
  