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 F(C):
      S = "".join(('(' if i % 2 == 0 else ')') * C[i] for i in range(len(C)))
      r = 0
      for i in range(len(S)):
          o = 0
          if S[i] == '(':
              for s in S[i:]:
                  if s == '(':
                      o += 1
                  else:
                      if o > 0:
                          o -= 1
                          if o == 0:
                              r += 1
                      else:
                          break
  
      return r
              
  
  n = int(input())
  C = list(map(int, input().split(' ')))
  
  n = n - (n % 2)
  count, before = 0, []
  for i in range(0, n, 2):
      v = min(C[i], C[i + 1])
      count += v
  
      e = C[i] - C[i + 1]
      if e == 0:
          if len(before) > 0:
              before.append(before[-1])
          else:
              before.append(0)
      elif e > 0:
          if len(before) > 0:
              before.append(before[-1] + e)
          else:
              before.append(e)
      else: # e < 0
          if len(before) > 0:
              before.append(before[-1])
  
          while len(before) > 0 and e < 0:
              c = 0
              while len(before) > 1 and before[-1] == before[-2]:
                  before.pop()
                  c += 1
              count += c*(c + 1)//2
  
              b = before[-1] - (before[-2] if len(before) > 1 else 0)
              x = min(abs(e), b)
              before[-1] -= x
              count += x
              e += x
              if x == 0:
                  break
  
          if e == 0:
              pass
          else:
              before = []
  
  while len(before) > 0:
      c = 0
      while len(before) > 1 and before[-1] == before[-2]:
          before.pop()
          c += 1
      count += c*(c + 1)//2
      before.pop()
  
  print(count)
  
  
  
      
              
  
  
  