#include<bits/stdc++.h>
#include<algorithm>
#define lint long long int
#define ldouble long double
using namespace std;

int main(){
    lint a,b,c;
    c=-1;
    
    cin>>a>>b;
    for(lint i=1;i<a+b;i++){
        if((a+b)%i==0){
            c=i;
            break;
        }
    }
    cout<<c<<endl;
    return 0;
}