#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <cmath>

#define FORR(i,b,e) for(int i=(b);i<(int)(e);++i)
#define FOR(i,e) FORR(i,0,e)
#define ALL(c) begin(c), end(c)
#define dump(var) cerr << #var ": " << var << "\n"
#define dumpc(con) for(auto& e: con) cerr << e << " "; cerr<<"\n"

using namespace std;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    double xa, ya, xb, yb;
    cin >> xa >> ya >> xb >> yb;

    cout << fixed << setprecision(8) << (yb + (ya-yb) / (xa+xb) * xb) << endl;
    
    return 0;
}