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

def gcd(A,B):
    if B>A:
        A,B=B,A
    while A%B!=0:
        A,B=B,A%B
    return B

if gcd(A,B)!=1:
    print(-1)
    exit()

L=[0 for i in range(A*B+1)]
L[0]=1
for i in range(B+1):
    for j in range(A+1):
        if A*i+B*j<=A*B:
            L[A*i+B*j]=1
print(len(L)-sum(L))