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: from bisect import insort,bisect_right,bisect_left
  from sys import stdout, stdin, setrecursionlimit
  from heapq import heappush, heappop, heapify 
  from io import BytesIO, IOBase
  from collections import *
  from itertools import *
  from random import * 
  from string import *
  from queue import *
  from math import *
  from re import *
  from os import *
  # sqrt,ceil,floor,factorial,gcd,log2,log10,comb
  
  ####################################---fast-input-output----#########################################
  
  
  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 = read(self._fd, max(fstat(self._fd).st_size, 8192))
              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 = read(self._fd, max(fstat(self._fd).st_size, 8192))
              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:
              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")
  
  
  stdin, stdout = IOWrapper(stdin), IOWrapper(stdout)
  graph, mod, szzz =  {}, 10**9 + 7, lambda: sorted(zzz())
  def getStr(): return input()
  def getInt(): return int(input())
  def listStr(): return list(input())
  def getStrs(): return input().split()
  def isInt(s): return '0' <= s[0] <= '9'
  def input(): return stdin.readline().strip()
  def zzz(): return [int(i) for i in input().split()]
  def output(answer, end='\n'): stdout.write(str(answer) + end)
  def lcd(xnum1, xnum2): return (xnum1 * xnum2 // gcd(xnum1, xnum2))
  
  
  def getPrimes(N = 10**5):
      SN = int(sqrt(N))
      sieve = [i for i in range(N+1)]
      sieve[1] = 0
      for i in sieve:
          if i > SN:
              break
          if i == 0:
              continue
          for j in range(2*i, N+1, i):
              sieve[j] = 0
      prime = [i for i in range(N+1) if sieve[i] != 0] 
      return prime
  def primeFactor(n,prime=getPrimes()):
      lst = []
      mx=int(sqrt(n))+1
      for i in prime:
          if i>mx:break
          while n%i==0:
              lst.append(i)
              n//=i
      if n>1:
          lst.append(n)
      return lst    
  
  dx = [-1, 1, 0, 0, 1, -1, 1, -1]
  dy = [0, 0, 1, -1, 1, -1, -1, 1]
  daysInMounth = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
  
  
  #################################################---Some Rule For Me To Follow---#################################
  """
      --instants of Reading problem continuously try to understand them.
  
      --Try & again try, maybe you're just one statement away!
  
  """
  ##################################################---START-CODING---###############################################
  
  
  
  
  n = getInt()
  ans= 0
  arr = zzz()
  
  for i in range(0,n,2):
      prev=arr[i]
      cnt=prev-1
      for j in range(i+1,n):
          if j%2:
              x=prev-arr[j]
              t=min(cnt,prev)-max(x,0)
              if t>=0:
                  ans+=t+1
              prev=x
              if cnt>x:
                  cnt=x
          else:
              prev+=arr[j]
  
  print(ans)