#include <bits/stdc++.h>
#include<iostream>
#include<iomanip>
#include<string>
#include<algorithm>
#include<vector>
#include<set>
#include<list>
#include<queue>
#include<math.h>
#include<bitset>
using ll = long long;
using namespace std;

int main()
{
    int xa, xb, ya, yb;
    double d, f, ans;
    cin >> xa >> ya >> xb >> yb;
    d = yb - ya;
    ans = ya + d*xa/(xa+xb);
    cout << ans << endl;
}