#include <iostream>
using namespace std;
int main(){
        int x,y;
        cin >> x >> y;
        if(x==y){
                cout<<0<<endl;
                return 0;
        }
        if(x%y==0 || y%x==0){
                int counter=0;
                if(x<y){
                        counter++;
                        swap(x,y);
                }
                counter+=(x-y)/2;
                cout<<counter<<endl;
        }
        else
                cout<<-1<<endl;
        return 0;
}