#include<iostream>
#include<string>
#include<vector>
#include <algorithm>
#include <cmath>
using namespace std;
int n, m, cnt;
bool flag = true;
int main() {    
    cin >> n >> m;
    if((n!=1&&m==1) || (n==3&&m==2) || n-m > 3 ) {
        flag = false;
    }else{
        if(n==m) cout << 1 << endl;
        else cout << 2 << endl;
    }
    if(!flag) cout << -1 << endl;
    return 0;
}