#include<iostream>
#include<iomainp>
using namespace std;

int main() {
    double profit,principal;
    cin>>profit>>principal;
    
    try {
        if(principal==0) {
            throw principal;
        }
        double result=profit/principal;
        cout<<fixed<<setprecision(2)<<result;
    }
    catch(...){
        cout<<-1;
    }
    return 0;
}