#include <stdio.h>

int main() {
	int xa, ya, xb, yb;
	scanf("%d %d %d %d", &xa, &ya, &xb, &yb);

	double yp = (double)(yb - ya) / (xb + xa) * xa + ya;

	printf("%.7lf\n", yp);
	return 0;
}