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

def gcd(a,b):
    if b == 0:
        return a
    return gcd(b,a%b)
if gcd(A,B) > 1:
    print(-1)
    exit(0)

T = [1 for k in range(2*A*B+1)]
for x in range(B):
    for y in range(A):
        T[A*x+B*y] = 0
print(T[:A*B].count(1))