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.*;
  import java.util.*;
  
  public class MainClass {
      public static void main(String[] args) {
          Reader in = new Reader(System.in);
          StringBuilder stringBuilder = new StringBuilder();
          int t = 1;
          while (t-- > 0) {
              int n = in.nextInt();
              long[] a = new long[n];
              for (int i = 0; i < n; i++) {
                  a[i] = in.nextLong();
              }
              long[][][] count = new long[n][n][2];
              for (int i = 0; i < n; i++) {
                  int j = i;
                  long open = 0L;
                  long closed = 0L;
                  while (j < n) {
                      if (j % 2 == 0) {
                          open += a[j];
                      } else {
                          long bal = Math.min(open, a[j]);
                          closed += a[j] - bal;
                          open -= bal;
                      }
                      count[i][j] = new long[]{open, closed};
                      j++;
                  }
              }
              long ans = 0L;
              for (int i = 0; i < n; i += 2) {
                  for (int j = i; j < n; j++) {
                      if (i % 2 != j % 2) {
                          if (i == j - 1) {
                              //System.out.println(i + " " + j + " " + Math.min(a[i], a[j]));
                              ans += Math.min(a[i], a[j]);
                          } else {
                              long open = count[i + 1][j - 1][0];
                              long closed = count[i + 1][j - 1][1];
                              long currOpen = a[i];
                              long currClosed = a[j];
                              currOpen -= closed;
                              currClosed -= open;
                              if (currOpen >= 0 && currClosed >= 0) {
                                  long curr = Math.min(currClosed, currOpen) + 1;
                                  //System.out.println(i + " " + j + " " + curr);
                                  ans += Math.min(currOpen, currClosed) + 1;
                              }
                          }
                      }
                  }
              }
              System.out.println(ans);
          }
      }
  }
  class Reader {
      public BufferedReader reader;
      public StringTokenizer tokenizer;
  
      public Reader(InputStream stream) {
          reader = new BufferedReader(new InputStreamReader(stream), 32768);
          tokenizer = null;
      }
  
      public String next() {
          while (tokenizer == null || !tokenizer.hasMoreTokens()) {
              try {
                  tokenizer = new StringTokenizer(reader.readLine());
              } catch (IOException e) {
                  throw new RuntimeException(e);
              }
          }
          return tokenizer.nextToken();
      }
  
      public int nextInt() {
          return Integer.parseInt(next());
      }
  
      public long nextLong() {
          return Long.parseLong(next());
      }
  
  }