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;
  inline int read() {
    register int s = 0, f = 0;
    register char ch = getchar();
    while (!isdigit(ch)) {
      f |= (ch == '-');
      ch = getchar();
    }
    while (isdigit(ch)) {
      s = (s << 1) + (s << 3) + (ch ^ 48);
      ch = getchar();
    }
    return f ? -s : s;
  }
  inline void write(int x) {
    if (x < 0) {
      putchar('-');
      x = -x;
    }
    if (x > 9) {
      write(x / 10);
    }
    putchar(x % 10 + '0');
  }
  long long a[1005], n, ans;
  signed main() {
    cin >> n;
    for (auto i = 1; i <= n; i += 1) {
      cin >> a[i];
    }
    for (auto i = 1; i <= n; i += 2) {
      long long sum = 0;
      long long mn = a[i] - 1;
      for (auto j = i + 1; j <= n; j += 2) {
        sum += a[j - 1];
        sum -= a[j];
        if (sum < 0) {
          ans += min(a[j] + sum + 1, mn + 1);
          break;
        } else {
          ans += min(a[j], max(0LL, mn - sum + 1));
          if (j == i + 1) {
            mn = min(a[i], sum);
          } else {
            mn = min(mn, sum);
          }
        }
      }
    }
    cout << ans;
    return 0;
  }
  