#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll calc(ll m, ll n, ll sqn = -1) {
	ll d = m / sqrt(n);
	if (sqn != -1) {
		d = m / sqn;
	}
	return m - d;
}
int main () {
	ll N, M;
	cin >> N >> M;
	if (N == 1) {
		puts("-1");
		return 0;
	}
	ll sqn = 0;
	while (sqn * sqn < N) sqn ++;
	if (sqn * sqn != N) sqn = -1;
	ll ok = (ll)1e7, ng = -1;
	while (abs(ok - ng) > 1) {
		ll mu = (ok + ng) / 2;
		if (calc(mu, N, sqn) >= M) {
			ok = mu;
		} else {
			ng = mu;
		}
	}
	cout << ok << endl;
}