#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>
using namespace std;

int main()
{
    //freopen("data.in", "rt", stdin);
    //freopen("data.out", "wt", stdout);
    int a, b, c, d;
    scanf("%d%d%d%d", &a, &b, &c, &d);
    cout <<fixed << setprecision(5)<< (double)a/(double)c +(double)b/(double)d<< '\n';


    return 0;
}