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
  inputt=sys.stdin.readline
  printt=sys.stdout.write
  
  import math
  import functools  # functools.reduce
  from collections import deque
  import heapq
  from queue import PriorityQueue
  
  def get():
      return inputt().split()
  def getint():
      return int(inputt())
  def getints():
      return map(int, inputt().split())
  
  n = getint()
  c = list(getints())
  a = []
  x = -1
  for cc in c:
      a.append([x, cc])
      if x == -1:
          a.append([0,0])
      x*=-1
  while 1:
      changed = False
      ac = None
      aa = []
      for i, xy in enumerate(a):
          x, y = xy
          if x == -1:
              ac = i
              acc = y
              aa = []
          elif x == 0:
              aa.append(y)
          elif x == 1:
              if ac != None:
                  m = min(acc, y)
                  a[ac][1] -= m
                  a[i][1] -= m
                  k = len(aa)
                  a[i-1][1] = (k*(k-1))//2 + m + sum(aa)
                  for j in range(i-k, i-1):
                      a[j][1] = 0
                  ac = None
                  changed = True
      if not changed:
          break
      a = list(filter(lambda x: x[1]!=0, a))
  
  cont = False
  aaaa = []
  aaab = 0
  for x in a:
      if x[0] != 0 and cont:
          mm = len(aaaa)
          aaab += sum(aaaa)+(mm*(mm-1))//2
          aaaa = []
          cont = False
      elif x[0] !=0:
          cont = False
      elif x[0] == 0:
          aaaa.append(x[1])
          cont = True
  if len(aaaa):
      mm = len(aaaa)
      aaab += sum(aaaa)+(mm*(mm-1))//2
  print(aaab)