#include <stdio.h>

int main() {
	double a, b, x, y;

	scanf("%lf %lf %lf %lf", &a, &b, &x, &y);
	
	if (x * (b / a) > y) printf("%.7lf\n", y + y * (a / b));
	else printf("%.7lf\n", x + x * (b / a));

	return 0;
}