#include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;

int main(){
	int n, m;
	cin >> n >> m;
	if(n%2==0){
		if(m>=n/2) cout << 2 << endl;
		else cout << -1 << endl;
	}else{
		if(n==m) cout << 1 << endl;
		else cout << -1 << endl;
	}
	return 0;
}