//https://ncode.syosetu.com/n4830bu/306/
#include <bits/stdc++.h>
using namespace std;

int main() {
    double xa, ya, xb, yb;
    cin >> xa >> ya >> xb >> yb;
    cout << fixed << setprecision(10) << ya + (yb - ya) * xa / (xb + xa) << endl;
}