a,b=map(int, input().split())
D=[0]*(a*b)
for i in range(a+1):
    for j in range(b+1):
        if b*i+a*j<=a*b:
            D[(b*i+a*j)-1]=1 
d=D.count(0)
import math
z=math.gcd(a,b)
if z==1:
    print(d)
else:
    print(-1)