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())
  arr=[ int(x) for x in input().split()]
  ans = [int(0) for x in range(n+1)]
  tempans = [int(0) for x in range(n+1)]
  arr.insert(0,0)
  for i in range(2, n+1, 2):  
    if arr[i]<arr[i-1]:
      ans[i]+=arr[i]
    else:
      ans[i]+=arr[i-1]
      temp=arr[i]
      ans[i]+=tempans[i-2]
      temp-=arr[i-1]
      j=i-2
      tt=temp
      temp=0
      while tempans[j] and tt>0 and j>0:
        if arr[j-1]>temp+tt+arr[j]:
          ans[i]+=tt
          temp=0
          tt=0
        else:
          if arr[j-1]>=(arr[j]+temp) :
            ans[i]+=arr[j-1]-(arr[j]+temp)
            if arr[j-1]-(arr[j]+temp):
              ans[i]+=tempans[j-2]
          temp+=arr[j]
          temp-=arr[j-1]
          if(temp<0):
            tt+=temp
            temp=0
            
        j-=2
    temp=0
    k=1
    j=i
    while k or (temp>0 and j>0):
      k=0
      temp+=arr[j]
      temp-=arr[j-1]
      j-=2
    if temp==0:
      tempans[i]=1+tempans[j]
    elif temp<0:
      tempans[i]=1
        
  
      
  answer=0
  for i in range(n+1):
    answer+=ans[i]
  
  # print(ans)
  # print(tempans)
  print(answer)
  
  