#include<iostream>
#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 (int) {
        cout << -1;
    }
    
    return 0;
}