INF = 10 ** 9
MOD = 10 ** 9 + 7
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)
from  copy import deepcopy
from bisect import bisect_left

def main():
    a,b = map(int,input().split())
    n = a + b
    ans = -1
    for i in range(1,n):
        if i * i > n:
            break
        if n%i == 0:
            c = i
            if c != a and c != b and (b + c)%a == 0 and (c + a)%b == 0:
                ans = c
                break
            c = n//i
            if c != a and c != b and (b + c)%a == 0 and (c + a)%b == 0:
                ans = c
                break
            
    print(ans)
if __name__ == '__main__':
    main()