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 random
  import collections
  import string
  import math
  import copy
  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)
  
  
  def input(): return sys.stdin.readline().rstrip("\r\n")
  
  
  # mo = 998244353
  mo = int(1e9+7)
  
  
  def exgcd(a, b):
      if not b:
          return 1, 0
      y, x = exgcd(b, a % b)
      y -= a//b * x
      return x, y
  
  
  def getinv(a, m):
      x, y = exgcd(a, m)
      return -1 if x == 1 else x % m
  
  
  def comb(n, b):
      res = 1
      b = min(b, n-b)
      for i in range(b):
          res = res*(n-i)*getinv(i+1, mo) % mo
          # res %= mo
      return res % mo
  
  
  def quickpower(a, n):
      res = 1
      while n:
          if n & 1:
              res = res * a % mo
          n >>= 1
          a = a*a % mo
      return res
  
  
  def dis(a, b):
      return abs(a[0]-b[0]) + abs(a[1]-b[1])
  
  
  def getpref(x):
      if x > 1:
          return (x)*(x-1) >> 1
      else:
          return 0
  
  
  def orafli(upp):
      primes = []
      marked = [False for i in range(upp+3)]
      prvs = [i for i in range(upp+3)]
      for i in range(2, upp):
          if not marked[i]:
              primes.append(i)
          for j in primes:
              if i*j >= upp:
                  break
              marked[i*j] = True
              prvs[i*j] = j
              if i % j == 0:
                  break
      return primes, prvs
  
  
  def lower_ord(c: str) -> int:
      return ord(c)-97
  
  
  def upper_ord(c: str) -> int:
      return ord(c) - 65
  
  
  def read_list():
      return [int(i) for i in input().split()]
  
  
  def read_int():
      s = input().split()
      if len(s) == 1:
          return int(s[0])
      else:
          return map(int, s)
  
  
  def ask(s):
      print(f"? {s}", flush=True)
  
  
  def answer(s):
      print(f"{s}", flush=True)
  
  import functools
  
  # primes, prvs = orafli(100010)
  
  import itertools
  from fractions import Fraction
  
  import random
  # A = list(map(list,(zip(*A))))
  
  
  def lowbit(x):
      return -x&x
  
  def pref(x):
      return x*(x+1)>>1
  
  def cum(sx, mx):
      return (sx+mx)*(mx-sx+1)>>1
  
  
  
  def solve():
      n = read_int()
      li = read_list()
  
      dp = {0:1}
      ans = 0
      px = 0
      for p, i in enumerate(li):
          if p&1:
              cl = 0
              ato = i
              pf = 0
              cr = 0
              for q in range(p-1, -1, -1):
                  if q&1:
                      cr += li[q]
                      # clo = cl
                      # cl-=li[q]
                      # lb = max(cl, 1)
                      # rb = min(ato, clo)
                      # if not ato and cl == 0:
                      #     ans+=1
                      # if rb-lb+1>0:
                      #     ans += rb - lb + 1
                      #     ato -= rb - lb + 1
                      
                  else:
                      # clo = cl
                      # cl+=li[q]
                      cr -= li[q]
                      if cr<=0:
                          ans += min(ato, abs(cr))+1
                          # lb = max(clo, 0)
                          # rb = min(i, cl)
                          ato+=cr
                          cr = 0
                      # if not ato and cl == 0:
                      #     ans+=1
                      # if rb-lb+1>0:
                      #     ans += rb - lb + 1
                      #     ato -= rb - lb + 1
                      if ato<0:
                          break
              
              ans -= 1
              # ans += dp.get(i, 0)
              # px-=i
              # dp[px] = dp.get(px, 0) + 1
          # else:
          #     ans += dp.get(-i, 0)
          #     px+=i
          # dp[px] = dp.get(px, 0) + 1
      
          # dpp = {}
          # for k, v in dp.items():
          #     dpp[k+i] = max(dpp.get(k+i, 0), v)
          #     mk = max(0, k-i)
          #     dpp[mk] = max(dpp.get(mk, 0), (k-mk) + v)
          #     ans = max(ans, dpp[k+i], dpp[mk])
          # dp = dpp
      print(ans)
  
      
  
  
      
  # fi = open('C:\\cppHeaders\\CF2020.12.17\\test.data', 'r')
  # def input(): return fi.readline().rstrip("\r\n")
  # primes, prv = orafli(10001)
  # solve()
  # T = int(input())
  T = 1
  
  
  
  for ti in range(T):
      solve()
      # except:
          # traceback.print_exc()
  
  
  """
  6 5
  3 1 1
  4 3 2
  5 4 3
  6 5 4
  2 6 5
  
  
  """