#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#define rep(i,n) for(int i = 0;i < n;i++)
#define P(p) cout<<(p)<<endl;
using namespace std;
typedef long long ll;
int dx[] = { 0, 1, 0, -1 };
int dy[] = { -1, 0, 1, 0 };

int sttoi(std::string str) {
	int ret;
	std::stringstream ss; ss << str;
	ss >> ret;
	return ret;
}

ll gcd(ll a, ll b){
	if (b > a)swap(a, b); if (b == 0)return a; else{ return gcd(b, a%b); }
}

void solve() {
	double x1, x2, y1, y2;
	cin >> x1 >> y1 >> x2 >> y2;
	x2 = -x2;
	double k = (y1 - y2) / (x1 - x2);
	double b = y1 - k*x1;
	cout << setprecision(10) << fixed << b << endl;
}

int main() {
	solve();
	return 0;
}