#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

int main() {
    int x0, y0, x1, y1;
    cin >> x0 >> y0;
    cin >> x1 >> y1;

    x1 = -x1;
    double result = -1.0 * (double)(y1-y0) * x0 / (double)(x1-x0) + y0;
    printf("%08f\n", result);
}