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 os
  import sys
  from io import BytesIO, IOBase
  BUFSIZE = 8192
  class FastIO(IOBase):
      newlines = 0
      def __init__(self, file):
          self._fd = file.fileno()
          self.buffer = BytesIO()
          self.writable = 'x' in file.mode or 'r' not in file.mode
          self.write = self.buffer.write if self.writable else None
      def read(self):
          while True:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              if not b:break
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines = 0
          return self.buffer.read()
      def readline(self):
          while self.newlines == 0:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              self.newlines = b.count(b'\n') + (not b)
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines -= 1
          return self.buffer.readline()
      def flush(self):
          if self.writable:
              os.write(self._fd, self.buffer.getvalue())
              self.buffer.truncate(0), self.buffer.seek(0)
  class IOWrapper(IOBase):
      def __init__(self, file):
          self.buffer = FastIO(file)
          self.flush = self.buffer.flush
          self.writable = self.buffer.writable
          self.write = lambda s: self.buffer.write(s.encode('ascii'))
          self.read = lambda: self.buffer.read().decode('ascii')
          self.readline = lambda: self.buffer.readline().decode('ascii')
  sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
  input = lambda: sys.stdin.readline().rstrip('\r\n')
  
  def solve(n, c):
      l = [val for ind, val in enumerate(c) if ind % 2 == 0]
      r = [val for ind, val in enumerate(c) if ind % 2 == 1]
      ans = 0
      stack = [[0, 0]]
      for o, c in zip(l, r):
          ans += min(o, c)
          if o == c:
              ans += stack[-1][1]
              stack[-1][1] += 1
          elif o > c:
              stack.append([o - c, 1])
          else:
              ans += stack[-1][1]
              excess = c - o
              while excess > 0:
                  if len(stack) == 0:
                      stack = [[0, 0]]
                      break
                  if excess < stack[-1][0]:
                      ans += excess
                      stack[-1][0] -= excess
                      stack[-1][1] = 1
                      break
  
                  if excess == stack[-1][0]:
                      ans += excess
                      if len(stack) == 1:
                          stack[-1][0] -= excess
                          stack[-1][1] = 1
                      else:
                          stack.pop()
                          ans += stack[-1][1]
                          stack[-1][1] += 1
                      break
  
                  if excess > stack[-1][0]:
                      if len(stack) == 1:
                          ans += stack[-1][0]
                          excess -= stack[-1][0]
                          stack.pop()
                          stack.append([0, 0])
                          break
                      else:
                          ans += stack[-1][0]
                          excess -= stack[-1][0]
                          stack.pop()
                          ans += stack[-1][1]
                          stack[-1][1] += 1
      return ans
  
  # def gen(n):
  #     import random
  #     ret = ''
  #     for i in range(n):
  #         x = random.randint(1, 100)
  #         if i % 2 == 0:
  #             ch = '('
  #         else:
  #             ch = ')'
  #         ret += ch * x
  #     return ret
  
  # def isvalid(s, i, j):
  #     c = 0
  #     for a in range(i, j + 1):
  #         if s[a] == '(':
  #             c += 1
  #         else:
  #             c -= 1
  #         if c < 0:
  #             return False
  #     if c == 0:
  #         return True
  #     else:
  #         return False
  
  # def ans(s):
  #     ans = 0
  #     for i in range(len(s)):
  #         for j in range(i, len(s)):
  #             if isvalid(s, i, j):
  #                 ans += 1
  #     return ans
  
  # def compress(s):
  #     arr = [1]
  #     for i in range(1, len(s)):
  #         if s[i] == s[i - 1]:
  #             arr[-1] += 1
  #         else:
  #             arr.append(1)
  #     return arr
  
  
  if __name__ == '__main__':
      # n = 20
      # s = gen(n)
      # s = "()(()"
      n = int(input())
      c = [*map(int, input().split())]
  
      # c = compress(s)
      # correct_ans = ans(s)
      my_ans = solve(n, c)
      print(my_ans)
      # if correct_ans != my_ans:
          # print(n)  
          # print(s)  
          # print(c)  
          # print(correct_ans)  
          # print(my_ans)  