#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<iomanip>
#include<set>
#include<numeric>
#include<sstream>
#include<random>
#include<cassert>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rrep(i, st, n) for (int i = st; i < n; ++i)
using pii = pair<int, int>;
const int inf = 1e9 + 7;
int dy[] = {0, 0, -1, 1, -1, 1, -1, 1};
int dx[] = {1, -1, 0, 0, -1, 1, 1, -1};
#define ceil(a, b) a / b + !!(a % b)

int main() {
    int a, b; cin >> a >> b;
    int ans = inf;
    rrep(i, 1, sqrt(a + b) + 1) {
        if (i == a || i == b) continue;
        if ((a + b) % i == 0) {
            if ((a + i) % b == 0 && (b + i) % a == 0) {
                ans = min(ans, i);
            }
            int j = (a + b) / i;
            if ((a + j) % b == 0 && (b + j) % a == 0) {
                ans = min(ans, j);
            }
        }
    }
    if (ans != inf) cout << ans << endl;
    else cout << -1 << endl;
}