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()))
  
  s = []
  
  answer = 0
  
  for i in range(n):
      if i % 2 == 0:
          if len(s) == 0 or s[-1][0] != '(':
              s.append(['(', a[i]])
          else:
              s[-1][1] += a[i]
      else:
          while len(s) > 0:
              if len(s) > 0 and s[-1][0] == '(':
                  if s[-1][1] <= a[i]:
                      a[i] -= s[-1][1]
                      s[-1][0] = '()'
                  else:
                      s[-1][1] -= a[i]
                      s.append(['()', a[i]])
                      a[i] = 0
  
              if a[i] == 0:
                  break
  
              accu = 0
              while len(s) > 0 and s[-1][0] == '()':
                  answer += s[-1][1] + accu
                  s.pop()
                  accu += 1
  
  accu = 0
  while len(s) > 0:
      if s[-1][0] == '()':
          answer += s[-1][1] + accu
          accu += 1
          s.pop()
      else:
          accu = 0
          s.pop()
  
  print(answer)
  