/* -*- coding: utf-8 -*-
 *
 * 816.cc:  No.816 Beautiful tuples - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const long long LINF = 1LL << 62;

/* typedef */

typedef long long ll;

/* global variables */

/* subroutines */

inline bool beautiful(ll a, ll b, ll c) {
  return (a + c) % b == 0 && (b + c) % a == 0;
}

/* main */

/*
  a+c=xb, b+c=ya (a<b,x>0,y>0)
  b-a=ya-xb=xb-ya
 */

int main() {
  ll a, b;
  scanf("%lld%lld", &a, &b);

  ll ab = a + b, minc = LINF;
  for (int c = 1; c * c <= ab; c++)
    if (ab % c == 0) {
      ll d = ab / c;
      if (a != c && b != c && beautiful(a, b, c) && minc > c) minc = c;
      if (a != d && b != d && beautiful(a, b, d) && minc > d) minc = d;
    }

  printf("%lld\n", (minc >= LINF) ? -1 : minc);
  return 0;
}