#include <iostream>
#include <cstdio>

using namespace std;

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

	cin >> xa >> ya >> xb >> yb;
	xb *= -1;

	double ans = (ya - yb) / (xa - xb) * (-xa) + ya;
	printf("%.9f\n", ans);
	return 0;
}