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
  
  n=int(input())
  C=list(map(int,input().split()))
  
  S=[0]
  for i in range(n):
      c=C[i]
      if i%2==0:
          S.append(S[-1]+c)
      else:
          S.append(S[-1]-c)
  
  X=[]
  for s in S:
      X.append(s-1)
      X.append(s)
      X.append(s+1)
  
  X=sorted(set(X))
  D={X[i]:i for i in range(len(X))}
  
  Y=[[] for i in range(len(X))]
  
  now=0
  for i in range(n):
      c=C[i]
      if i%2==0:
          for k in range(D[now],D[now+c]):
              Y[k].append(1)
          now+=c
      else:
          for k in range(D[now],D[now-c],-1):
              Y[k].append(-1)
          now-=c
  
  if n%2==0:
      Y[D[now]].append(1)
  else:
      Y[D[now]].append(-1)
  
  ANS=0
  
  for i in range(len(Y)-1):
  
      count=0
      K=0
      for y in Y[i]:
          if y==1:
              count+=1
          else:
              K+=count*(count+1)//2
              count=0
      if count!=0:
          K+=count*(count-1)//2
  
      ANS+=K*(X[i+1]-X[i])
  
  print(ANS)
          
          
      
  