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>
  inline void read(T &x) {
    x = 0;
    char c = getchar(), f = 0;
    for (; c < 48 || c > 57; c = getchar())
      if (!(c ^ 45)) f = 1;
    for (; c >= 48 && c <= 57; c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
    f ? x = -x : x;
  }
  template <typename T, typename... L>
  inline void read(T &x, L &...l) {
    read(x), read(l...);
  }
  int n;
  long long s[10005];
  int main() {
    read(n);
    for (int i = 1; i <= n; i++)
      read(s[i]), s[i] = i & 1 ? s[i] : -s[i], s[i] += s[i - 1];
    long long rs = 0;
    for (int i = 1; i <= n; i += 2) {
      long long mn = 1e18;
      for (int j = i + 1; j <= n; j++) {
        long long qaq = s[j - 1] - s[i];
        if (!(j & 1) && max(1ll, -mn) <= s[i] - s[i - 1])
          rs += max(0ll, min(abs(s[i] - s[i - 1]), abs(s[j] - s[j - 1]) - qaq) -
                             max(1ll, -mn) + 1);
        mn = min(mn, s[j] - s[i]);
      }
    }
    return printf("%lld\n", rs), 0;
  }
  