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: # t = int(input())
  # for iii in range(t):
  n = int(input())
  q = dict()
  ans = 0
  a = list(map(int, input().split()))
  for i in range(1, n, 2):
      cnt = a[i]
      max_cnt = a[i]
      for j in range(i + 1, n):
          if j % 2 == 0:
              cnt -= a[j]
              q[(i, j)] = [cnt, max_cnt]
          else:
              cnt += a[j]
              max_cnt = max(max_cnt, cnt)
  for i in range(0, n, 2):
      for j in range(i + 1, n, 2):
          if i == j - 1:
              ans += min(a[i], a[j])
          else:
              l = a[i]
              r = a[j]
              p = q[(i + 1, j - 1)]
              # print(i, j, p)
              l -= p[1]
              p[0] -= p[1]
              r += p[0]
              # print(l, r)
              if l >= 0 and r >= 0:
                  ans += min(l, r) + 1
  print(ans)
  
  