#include <iostream>
#include <string>
#include <cstdio>
#include <iomanip>

using namespace std;

int main(void) {
	double xa, xb, ya, yb;
	cout << std::fixed << setprecision(11);
	cin >> xa >> ya >> xb >> yb;
	if (xa * xb > 0) xa *= -1;
	cout << ya - (yb - ya) / (xb - xa)*xa << endl;
}