#include <bits/stdc++.h>

int main() {
	int x0, y0, x1, y1;
	scanf("%d%d%d%d", &x0, &y0, &x1, &y1);
	printf("%.7f\n", y0 + (double) (y1 - y0) * x0 / (x0 + x1));
	return 0;
}