#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;

int main()
{
	double xa, ya, xb, yb;
	cin >> xa >> ya >> xb >> yb;

	double a = abs(yb -ya) / abs(xb + xa);
	double b = yb - xb*a;
	printf("%.10f\n",b);
	return 0;
}