#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define coutf(f) cout << fixed << setprecision(f)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()

int main() {
	int n, k;
	cin >> n >> k;
	if (n < k) {
		cout << -1 << endl;
	}
	else {
		cout << k - 1 << endl;
	}
	return 0;
}