import sys
input=sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))

def main():
    mod=10**9+7
    import math
    
    a,b=MI()
    if min(a,b)==1:
        print(0)
        exit()
    
    if math.gcd(a,b)!=1:
        print(-1)
    else:
        S=set([])
        M=a*b
        for i in range(101):
            for j in range(101):
                temp=a*i + b*j
                if temp<M:
                   S.add(temp)
        print(M-len(S))


main()