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
  
  #comment these out later
  #sys.stdin = open("in.in", "r")
  #sys.stdout = open("out.out", "w")
  
  input = sys.stdin.readline
  
  def main():
  	#t = int(input())
  	t = 1
  
  	for _ in range(t):
  		n = int(input())
  		ar = list(map(int, input().split()))
  
  		
  		signs = [0]
  
  		#pre = [0]
  
  		for i, x in enumerate(ar):
  			if i%2:
  				signs.append(signs[-1]-x)
  			else:
  				signs.append(signs[-1]+x)
  
  			#pre.append(pre[-1] + signs[-1])
  		
  
  		ans = 0
  
  		for l in range(0, n, 2):
  			for r in range(l+1, n, 2):
  
  				left = ar[l]
  				right = ar[r]
  
  				if r-l == 1:
  					ans += min(left, right)
  					continue
  
  				stuff = signs[r] - signs[l+1]
  
  				#print(l, r, signs[l+2:r+1], signs[l+1])
  
  				minleft = min(signs[l+2:r+1]) - signs[l+1]
  
  				minleft = max(-minleft, 1)
  
  				equivmin = stuff + minleft
  
  				#print(l, r, left, right, minleft, equivmin, stuff)
  
  				ans += max(min(left-minleft+1, right-equivmin+1), 0)
  
  				#print(ans)
  
  		print(ans)
  
  
  
  
  
  
  
  main()