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: """
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  """
  
  n = int(input())
  aa = list(map(int, input().split()))
  ans = 0
  stk = []
  for i in range(n):
    if not stk:
      stk.append([0, 1])
    if i & 1:
      while aa[i] and stk:
        if stk[-1][1] == 0:
          stk.pop()
        if stk:
          now = stk.pop()
          sub = min(now[0], aa[i])
          aa[i] -= sub
          now[0] -= sub
          ans += sub
          if now[0]:
            stk.append(now)
          if stk and stk[-1][1] == 0:
            ans += stk[-1][0]
      if aa[i]:
        stk.append([0, 1])
      else:
        if not stk:
          stk.append([0, 0])
        if stk[-1][1] == 0:
          stk[-1][0] += 1
        else:
          stk.append([1, 0])
    else:
      if stk[-1][1] == 0:
        stk.append([0, 1])
      stk[-1][0] += aa[i]
    # print(i, stk)
  print(ans)