output_description: Print an integer representing the minimum number of times Takahashi will press
  the buttons.
  
  * * *
input_description: The input is given from Standard Input in the following format:
  
      
      
      N
      A_1 B_1
      :
      A_N B_N
attempt: import sys
  import os
  
  def file_input():
      f = open('AGC009/input.txt', 'r')
      sys.stdin = f
  
  def main():
      #file_input()
      N=int(input())
      A=[]
      B=[]
      for i in range(N):
          tmp=list(map(int, input().split()))
          A.append(tmp[0])
          B.append(tmp[1])
  
      cnt=0
      for j in range(N):
          # while (A[N-1-j]+cnt)%B[N-1-j]!=0:
          # cnt+=1
          tmp=A[N-1-j]+cnt
          if not tmp==0:
              # cnt=B[N-1-j]
              elif tmp<B[N-1-j]:
                  cnt+=B[N-1-j]-tmp
              elif tmp%B[N-1-j]!=0:
                  cnt+=B[N-1-j]-tmp%B[N-1-j]
  
      print(cnt)
  
  if __name__ == '__main__':
      main()
  
problem_description: Statement
  
  There are an integer sequence A_1,...,A_N consisting of N terms, and N
  buttons. When the i-th (1 ≦ i ≦ N) button is pressed, the values of the i
  terms from the first through the i-th are all incremented by 1.
  
  There is also another integer sequence B_1,...,B_N. Takahashi will push the
  buttons some number of times so that for every i, A_i will be a multiple of
  B_i.
  
  Find the minimum number of times Takahashi will press the buttons.
samples: [{"input": "3\n    3 5\n    2 7\n    9 4", "output": "7\n    \n\nPress the first button twice, the second button twice and the third button\nthree times.\n\n* * *"}, {"input": "7\n    3 1\n    4 1\n    5 9\n    2 6\n    5 3\n    5 8\n    9 7", "output": "22"}]