#include <bits/stdc++.h>
#define PI 3.14159265359
#define NIL -1
using namespace std;
const int64_t MOD = 1e9 + 7;

int main() {
	int N,K;
	cin >> N >> K;

	if (N < K) {
		cout << -1 << endl;
	}
	else {
		cout << K - 1 << endl;
	}
}