#include <iostream>
#include <algorithm>
#include <vector>
#include <cstring>
#include <string>
#include <math.h>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#define ll long long
#define pii pair<ll, ll>
#define pb push_back
#define all(v) (v).begin(), (v).end()
#define INF 1e15+7
#define eps 1e-8
#define DIV 998244353
#define pi 3.14159265

using namespace std;

double sx, sy, gx, gy, res;

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

	cin >> sx >> sy >> gx >> gy;
	gy = -gy;
	res = -sy * (sx - gx) / (sy - gy) + sx;
	printf("%.6lf", res);
	return 0;
}
#include <bits/stdc++.h>
using namespace std;

int main()
{
    double Sx,Sy,Gx,Gy;
    cin >> Sx >> Sy >> Gx >> Gy;
    double ans=double((Gx*Sy+Gy*Sx)/(Sy+Gy));
    cout << setprecision(10) << double(ans) << endl;
}