// editor1
#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    double pr,p;
    cin>>pt>p;
    try{
        if(p==0){
            throw 1;
        }
        double r=pr/p;
        cout<<fix<<setprecision(2)<<r;
    }
    catch(. . .)
    {
        cout<<-1;
    }
    return 0;
}