#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <bitset>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<pint> vpint;
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for (int i=(n)-1;i>=0;i--)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--)
int dx[4] = {1, 0, 0, -1};
int dy[4] = {0, 1, -1, 0};

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

	double rxa, rya;

	rxa = -xa;
	rya = ya;

	//printf("%f %f\n", rxa, rya);
	//直線aB
	double y;
	double katamuki = (yb - rya) / (xb - rxa);
	//printf("katamuki%f\n", katamuki);
	//y = katamuki * (x - xb) + yb;
	y = (-katamuki * xb) + yb;

	printf("%f\n", y);
	return 0;

}