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: """
  
      /$$$$$                       /$$            /$$$$$$                                                                        /$$
     |__  $$                      | $$           /$$__  $$                                                                      | $$
        | $$ /$$   /$$  /$$$$$$$ /$$$$$$        | $$  \__/  /$$$$$$   /$$$$$$        /$$$$$$/$$$$   /$$$$$$   /$$$$$$   /$$$$$$$| $$$$$$$
        | $$| $$  | $$ /$$_____/|_  $$_/        | $$$$     /$$__  $$ /$$__  $$      | $$_  $$_  $$ /$$__  $$ /$$__  $$ /$$_____/| $$__  $$
   /$$  | $$| $$  | $$|  $$$$$$   | $$          | $$_/    | $$  \ $$| $$  \__/      | $$ \ $$ \ $$| $$$$$$$$| $$  \__/| $$      | $$  \ $$
  | $$  | $$| $$  | $$ \____  $$  | $$ /$$      | $$      | $$  | $$| $$            | $$ | $$ | $$| $$_____/| $$      | $$      | $$  | $$
  |  $$$$$$/|  $$$$$$/ /$$$$$$$/  |  $$$$/      | $$      |  $$$$$$/| $$            | $$ | $$ | $$|  $$$$$$$| $$      |  $$$$$$$| $$  | $$
   \______/  \______/ |_______/    \___/        |__/       \______/ |__/            |__/ |__/ |__/ \_______/|__/       \_______/|__/  |__/
  
  
  
  """
  n = int(input())
  a = list(map(int, input().split()))
  if n & 1:
      n -= 1
  ans = 0
  for i in range(0,n,2):
      now = low = a[i]-a[i+1]
      ans += min(a[i], a[i+1])
      for j in range(i + 2, n, 2):
          now += a[j] - a[j+1]
          if now >= 0:
              tmp = a[i] - now
              ans += max(min(low-now, tmp, a[j+1]), -1)+1
          else:
              tmp = a[j+1]+now
              ans += max(min(low, a[i], tmp), -1)+1
          low = min(low, now)
          if low < 0:
              break
  print(ans)
  
  
  
  