import math
def my_lcm(x, y):
    return (x * y) // math.gcd(x, y)

A,B = map(int,input().split())
if math.gcd(A,B)>1:
    print(-1)
    exit()

ans = 0
lcm = my_lcm(A,B)
L=[0]*(lcm+1)
for j in range(101):
    for k in range(101):
        if A*j+B*k<=lcm:
            L[A*j+B*k]=1
print(len(L)-sum(L))