#include <iostream>
#define ll long long
#define int ll
using namespace std;

signed main()
{
    int x, y;
    cin >> x >> y;
    int ans = -1;
    if (y >= 2 * x - 1) {
        ans = x;
    }
    else {
        int m = y % x;
        int n = x - 1;
        ans = 0;
        for (int i = m; i >= 1; i--) {
            if (n <= 0) break;
            int k = n / i;
            n -= k * i;
            ans += (x + i) * k;
        }
        ans = ans - ans % x;
        if (m == 0) {
            ans = -1;
        }
    }
    cout << ans;
}