A,B=map(int,input().split())

import math

if math.gcd(A,B)!=1:
    print(-1)
else:
    ANS=[0]*(A*B+1)

    for i in range(100):
        for j in range(100):
            if A*i+B*j<=A*B:
                ANS[A*i+B*j]=1

    print(ANS.count(0))