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;
  long long solve(vector<int>& c, int n) {
    long long res = 0;
    for (int i = 0; i + 1 < n; i += 2) {
      long long s = c[i];
      long long mc = 0;
      bool fail = false;
      for (int j = i + 1; j < n && !fail; j += 2) {
        if (j > i + 1) {
          mc += c[j - 1];
        }
        long long e = c[j];
        if (e >= mc) {
          e -= mc;
          mc = 0;
          if (j == i + 1)
            res += min(s, e);
          else
            res += min(s + 1, e + 1);
        } else {
          mc -= e;
          e = 0;
        }
        if (s < e) fail = true;
        s -= e;
      }
    }
    return res;
  }
  int main(int argc, const char* argv[]) {
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> c(n);
    for (int i = 0; i < n; ++i) {
      cin >> c[i];
    }
    cout << solve(c, n) << "\n";
    return 0;
  }
  