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: //package com.company.codeforces.v210829;
  
  import java.io.BufferedReader;
  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.FileReader;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.List;
  import java.util.Set;
  import java.util.StringTokenizer;
  import java.util.stream.Collectors;
  import java.util.stream.IntStream;
  
  public class C {
      FastScanner in;
      PrintWriter out;
  
      public static void main(String[] args) {
          new C().solve();
      }
  
      private void solve() {
          in = new FastScanner(System.in);
          out = new PrintWriter(System.out);
  
          out.println(solveCase());
          out.flush();
      }
  
      private long solveCase() {
          int n = in.nextInt();
          List<Long> c = IntStream.range(0, n).mapToLong(i -> in.nextLong()).boxed().collect(Collectors.toList());
  
          long res = 0;
          for (int start = 0; start < n; start += 2) {
              long in = 0;
              long inAfterStart = 0;
              long minStartRequired = 0;
              for (int i = start; i < n; i++) {
                  if (i % 2 == 0) {
                      in += c.get(i);
                      if (i > start) inAfterStart += c.get(i);
                  } else {
                      if (inAfterStart + minStartRequired <= c.get(i)) {
                          res += Math.min(in, c.get(i)) - Math.max(1, inAfterStart + minStartRequired) + 1;
                      }
  
                     if (c.get(i) > in) {
                         break;
                     } else {
                         in -= c.get(i);
                         inAfterStart -= c.get(i);
                         minStartRequired = Math.max(minStartRequired, -inAfterStart);
                     }
                  }
              }
          }
          return res;
      }
  
      static class FastScanner {
  
          BufferedReader br;
          StringTokenizer st;
  
          public FastScanner(File f) {
              try {
                  br = new BufferedReader(new FileReader(f));
              } catch (FileNotFoundException e) {
                  e.printStackTrace();
              }
          }
  
          public FastScanner(InputStream f) {
              br = new BufferedReader(new InputStreamReader(f));
          }
  
          String next() {
              while (st == null || !st.hasMoreTokens()) {
                  String s = null;
                  try {
                      s = br.readLine();
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
                  if (s == null)
                      return null;
                  st = new StringTokenizer(s);
              }
              return st.nextToken();
          }
  
          boolean hasMoreTokens() {
              while (st == null || !st.hasMoreTokens()) {
                  String s = null;
                  try {
                      s = br.readLine();
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
                  if (s == null)
                      return false;
                  st = new StringTokenizer(s);
              }
              return true;
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
  
          double nextDouble() {
              return Double.parseDouble(next());
          }
      }
  
  }
  