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: import sys
  input=sys.stdin.readline
  INF=int(1e9)+7
  dx=[-1,0,1,0]
  dy=[0,1,0,-1]
  
  def solve():
      n=int(input())
      data=list(map(int,input().split()))
      remain=[]
      final=[0]*n
      dp=[0]*n
      for i in range(n):
          now=i%2
          if now==0:
              remain.append((i,data[i]))
          else:
              k=data[i]
              while k and remain:
                  idx,cost=remain.pop()
                  if cost<k:
                      dp[i]+=cost
                      k-=cost
                      if idx>0:
                          dp[i]+=final[idx-1] 
                  else:
                      if k==cost:
                          if idx>0:
                              final[i]=final[idx-1]+1
                              dp[i]+=final[idx-1]
                          else:
                              final[i]=1
                      else:
                          final[i]=1
                      dp[i]+=k
                      cost-=k
                      k=0
                      if cost: remain.append((idx,cost))
      print(sum(dp))
              
                      
                  
          
     
  t=1
  while t:
      t-=1
      solve()
  