import math
a,b = map(int,input().split())
gd = math.gcd(a,b)
if gd != 1:
    print(-1)
    exit()

li = [0]*(a*b+1)
for i in range(a+1):
    for j in range(b+1):
        tmp = b*i+a*j
        if tmp <= a*b:
            li[tmp] = 1
        
#print(li)
ans = 0
for i in range(a*b+1):
    if li[i] == 0:
        ans += 1
print(ans)