from math import gcd
A, B = map(int, input().split())
if gcd(A, B) > 1:
    print(-1)
    exit()
X = set()
for i in range(2 * B + 1):
    for j in range(2 * A + 1):
        c = i * A + j * B
        if c <= A * B:
            X.add(c)
print(A * B - len(X) + 1)