#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <string>
#include <queue>
#include <stack>
#include <math.h>
#include <set>
#define ALL(obj) (obj).begin(),(obj).end()
#define RALL(obj) (obj).rbegin(),(obj).rend()
#define P pair<int, int>

#define MOD 1000000007
#define INF 2147483647
#define NINF (-2147483647-1)
#define LLINF 9223372036854775807
using ll = long long;
using namespace std;

int main() {
    int A, B;
    cin >> A >> B;
    int a = min(A, B), b = max(A, B);
    int C = b - a;
    if ((C + b) % a == 0 && (a + b) % C == 0 && C != a && C!=b) {
        cout << C << endl;
    }
    else {
        C = a + b;
        if ((C + a) % b == 0 && (C + b) % a == 0) {
            cout << C << endl;
        }
        else {
            cout << -1 << endl;
        }
    }
    getchar(); getchar();
    return 0;
}