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;
  const long long int INF{(false ? (INT_MAX - 1) / 2 : (long long int)1e18 + 5)};
  long long int MOD = 1e9 + 7;
  void solve() {
    long long int n;
    cin >> n;
    vector<long long int> a(n);
    for (long long int i = 0; i < n; i++) cin >> a[i];
    long long int ans = 0;
    for (long long int i = 0; i < n; i++) {
      if (i % 2 == 0) {
        long long int l = a[i];
        if (i + 1 < n) {
          long long int r = min(l, a[i + 1]);
          ans += r;
          if (a[i + 1] - r > 0)
            continue;
          else {
            l -= r;
          }
        }
        long long int p = 0, p1 = 0, q = 0;
        for (long long int j = i + 2; j < n; j++) {
          if (j % 2 == 0)
            p += a[j], p1 = p - a[j];
          else {
            q = a[j];
            long long int r = min(q, p);
            p -= r;
            q -= r;
            if (p == 0) {
              ans += 1;
              long long int m = min(l, q);
              ans += m;
              l -= m;
              q -= m;
              if (q > 0) break;
            }
          }
        }
      }
    }
    cout << ans;
  }
  int main() {
    cout << setprecision(15);
    long long int t(1);
    while (t--) {
      solve();
      cout << endl;
    }
    return 0;
  }
  