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;
  template <typename T>
  bool chkmin(T &first, T second) {
    return second < first ? first = second, 1 : 0;
  }
  template <typename T>
  bool chkmax(T &first, T second) {
    return first < second ? first = second, 1 : 0;
  }
  template <typename T>
  void readint(T &first) {
    first = 0;
    int f = 1;
    char c;
    for (c = getchar(); !isdigit(c); c = getchar())
      if (c == '-') f = -1;
    for (; isdigit(c); c = getchar()) first = first * 10 + (c - '0');
    first *= f;
  }
  const int MAXN = 1005;
  int n;
  long long a[MAXN];
  long long sum[MAXN][MAXN], minp[MAXN][MAXN];
  int main() {
    readint(n);
    for (int i = 1; i <= n; ++i) readint(a[i]);
    if (n & 1) --n;
    long long res = 0;
    for (int i = 1; i <= n; ++i)
      for (int j = i; j <= n; ++j) {
        sum[i][j] = sum[i][j - 1] + a[j] * ((j & 1) ? 1 : -1);
        minp[i][j] = min(minp[i][j - 1], sum[i][j]);
      }
    for (int i = 1; i <= n; i += 2)
      for (int j = i + 1; j <= n; j += 2) {
        long long s = sum[i + 1][j - 1], m = -minp[i + 1][j - 1];
        long long l = m, r = a[i];
        chkmax(l, -s + 1);
        chkmin(r, a[j] - s);
        res += max(r - l + 1, 0ll);
      }
    cout << res << endl;
    return 0;
  }
  