#include <cmath>
#include <iostream>
#include <vector>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <bitset>
#include <algorithm>
#include <functional>
#include <utility>
#include <iomanip>

#define int long long int
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() )

using namespace std;

typedef pair<int, int> P;

const int INF = 1e15;
const int MOD = 1e9+7;

template <typename T>
using vector2 = vector<vector<T>>;
template <typename T>
vector2<T> initVec2(size_t n0, size_t n1, T e = T()){
    return vector2<T>(n0, vector<T>(n1, e));
}

template <typename T>
using vector3 = vector<vector<vector<T>>>;
template <typename T>
vector3<T> initVec3(size_t n0, size_t n1, size_t n2, T e = T()){
    return vector3<T>(n0, vector2<T>(n1, vector<T>(n2, e)));
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);

    int a, b;
    cin >> a >> b;

    int ans = -1;
    for(int x = 1; x * x <= a + b; x++){
        if(x == a || x == b){
            continue;
        }
        if((a + b) % x != 0){
            continue;
        }
        if((a + x) % b != 0){
            continue;
        }
        if((b + x) % a != 0){
            continue;
        }
        ans = x;
        break;
    }

    int x = a + b;
    if(ans == -1 && (a + b) % x == 0 && (a + x) % b == 0 && (b + x) % a == 0){
        ans = x;
    }
    cout << ans << endl;

    return 0;
}