// editor1
#include<iostream>
#include<iomainp>
using namespace std;
int main(){
    double profit,principal;
    cin>>profit>>principal;
    if(principal==0){
        cout<<"-1";
    }
    else{
        double answer=profit/principal;
        cout<<fixed<<setprecision(2)<<answer;
    }
    
    
    
    return 0;
}