#pragma comment (linker, "/STACK:526000000")
#define  _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

void init() {
	iostream::sync_with_stdio(false);
	cout << fixed << setprecision(100);
}
void solve(){
	int a, b;
	cin >> a >> b;
	int cnt = 0;
	for (int i = 1; i < a * b * 2 + 1; ++i) {
		int geko = 0;
		REP(q, 1000) {
			int hoge = i - a * q;
			if (hoge < 0) break;
			if (hoge % b == 0) {
				geko = 1;
				break;
			}
		}
		if (geko == 0) {
			cnt++;
			if (a * b <= i) {
				cout << -1 << endl;
				return;
			}
		}
	}
	cout << cnt << endl;
}
#undef int
int main() {
	//init();
	solve();
}