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