#include "bits/stdc++.h"
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000


int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int x1, y1;
	int x2, y2;
	cin >> x1 >> y1;
	cin >> x2 >> y2;
	x2 = -x2;
	int dx = x1 - x2;
	int dy = y1 - y2;
	double k = (double)dy / dx;
	cout << fixed;

	cout << setprecision(12) << y2 - k * x2 << endl;

    return 0;
}