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.util.*;
  public class Main {
      public static void main(String args[]) {
          Scanner s = new Scanner(System.in);
          int n = s.nextInt();
          long[] ar = new long[n];
          for (int i = 0; i < n; i++){
              ar[i] = s.nextLong();
          }
          long ans = 0;
          for (int i = 0; i < n; i+=2){
              long bal = 0, minbal = 0;
              for (int j = i+1; j < n; j++){
                  if (i%2 != j%2){
                      long left = -minbal, right = bal-minbal;
                      left = Math.max(left, 1);
                      right = Math.max(right, 1);
                      if (left <= ar[i] && right <= ar[j]){
                          ans += Math.min(ar[i]-left, ar[j]-right)+1;
                      }
                  }
                  if (j%2 == 0){
                      bal += ar[j];
                  } else {
                      bal -= ar[j];
                  }
                  minbal = Math.min(minbal, bal);
              }
          }
          System.out.println(ans);
      }
  }