MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
input = sys.stdin.readline
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)
from bisect import bisect_left,bisect_right
from math import gcd

def main():
    a,b = map(int,input().split())
    n = gcd(a,b)
    if n > 1:
        print(-1)
        return

    ans = [1] * (2 * a * b + 1)
    for i in range(2 * b + 1):
        for j in range(2 * a + 1):
            x = a * i + b * j
            if x <= 2 * a * b:
                ans[x] = 0
    
    print(sum(ans))

if __name__ =='__main__':
    main()