#include <iostream>
#include <fstream>
#include <vector>
#include <cstring>
#include <string>
#include <algorithm>
#include <iomanip>
#include <cmath>
using namespace std;

double x[2], y[2];

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

	for(int i = 0; i < 2; i++) {
		cin >> x[i] >> y[i];
	}

	double a = (y[1] - y[0]) / (-x[1] - x[0]);
	cout << setprecision(15) << y[1] + a * x[1] << endl;
}