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,sys
  from io import BytesIO,IOBase
  
  def main():
      n = int(input())
      c = list(map(int,input().split()))
      ans = 0
      for i in range(0,n,2):
          su,mini = 0,0
          for j in range(i+1,n):
              if not (j-i)&1:
                  su += c[j]
                  continue
              ans += max(0,min(c[i]+mini+(1 if mini else 0),c[j]-(su-mini)+(1 if su-mini else 0)))
              su -= c[j]
              mini = min(mini,su)
      print(ans)
  
  # Fast IO Region
  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")
  if __name__ == "__main__":
      main()