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: from sys import stdin, stdout
  ip = lambda : stdin.readline().rstrip("\r\n")
  ips = lambda : ip().split()
  out = lambda x, end='\n': stdout.write(str(x)+end)
  
  n = int(ip())
  a = list(map(int, ips()))
  ans = 0
  for i in range(n):
      no, nc, p = 0, 0, 0
      if i&1:
          continue
      x = a[i]
      if i+1<n:
          ans += min(a[i], a[i+1])
          x -= min(a[i], a[i+1])
      if i+1<n and a[i+1]>a[i]:
          continue
      y = 0
      f = 0
      for j in range(i+2, n):
          if j&1 == 0:
              y += a[j]
          else:
              z = a[j]
              if y>z:
                  y -= z
                  continue
              else:
                  z -= y
                  y = 0
                  ans += min(x, z)
                  ans += 1
                  if x>=z:
                      x -= z
                  else:
                      f = 1
          if f:
              break
  out(ans)