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: #include <bits/stdc++.h>
  using namespace std;
  int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    long long int n;
    cin >> n;
    vector<long long int> v(n);
    long long int i, j;
    for (i = 0; i < n; i++) {
      cin >> v[i];
    }
    long long int neu, ext;
    long long int rem;
    long long int ans = 0;
    bool con;
    for (i = 0; i < n; i = i + 2) {
      neu = v[i];
      ext = 0;
      rem = 0;
      for (j = i + 1; j < n; j++) {
        if (j % 2 == 1) {
          if (ext == 0) {
            rem = v[j];
            if (rem < neu) {
              ans += rem;
              neu -= rem;
            } else {
              if (rem == neu) {
                ans += rem;
                neu -= rem;
              } else {
                ans += neu;
                neu = 0;
                break;
              }
            }
          } else {
            if (v[j] < ext) {
              ext -= v[j];
            } else {
              if (v[j] == ext) {
                ext = 0;
                ans++;
              } else {
                rem = v[j] - ext;
                ext = 0;
                ans++;
                if (rem < neu) {
                  ans += rem;
                  neu -= rem;
                } else {
                  if (rem == neu) {
                    ans += rem;
                    neu -= rem;
                  } else {
                    ans += neu;
                    neu = 0;
                    break;
                  }
                }
              }
            }
          }
        } else {
          ext += v[j];
        }
      }
    }
    cout << ans << "\n";
  }
  