output_description: Print a positive integer not greater than 10^{18} that is a multiple of X but
  not a multiple of Y, or print -1 if it does not exist.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      X Y
attempt: #!/usr/bin/env python3
  #APC1 A
  
  import sys
  import math
  from bisect import bisect_right as br
  from bisect import bisect_left as bl
  sys.setrecursionlimit(1000000000)
  from heapq import heappush, heappop,heappushpop
  from collections import defaultdict
  from itertools import accumulate
  from collections import Counter
  from collections import deque
  from operator import itemgetter
  from itertools import permutations
  mod = 10**9 + 7
  inf = float('inf')
  def I(): return int(sys.stdin.readline())
  def LI(): return list(map(int,sys.stdin.readline().split()))
  
  a,b = LI()
  if a % b == 0:
    print(-1)
  tmp = 1
  while 1:
    if tmp % a == 0 and tmp % != 0:
      print(tmp)
      quit()
    tmp += 1
    
  
problem_description: Statement
  
  You are given positive integers X and Y. If there exists a positive integer
  not greater than 10^{18} that is a multiple of X but not a multiple of Y,
  choose one such integer and print it. If it does not exist, print -1.
samples: [{"input": "8 6", "output": "16\n    \n\nFor example, 16 is a multiple of 8 but not a multiple of 6.\n\n* * *"}, {"input": "3 3", "output": "-1\n    \n\nA multiple of 3 is a multiple of 3."}]