#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    int n, k;
    cin >> n >> k;

    cout << (n < k ? -1 : k - 1) << endl;

    return 0;
}