#include<stdio.h>

int main(){
	// 入力された値(割合)
	double ratio1, ratio2;
	scanf("%lf%lf",&ratio1,&ratio2);
	// 入力された値(全体)
	double overall1, overall2;
	scanf("%lf%lf",&overall1,&overall2);
	
	double ans = 0;
	if(overall1 / ratio1 * ratio2 <= overall2){
		ans = overall1 / ratio1 * ratio2 + overall1;
	}else{
		ans = overall2 / ratio2 * ratio1 + overall2;
	}
	
	printf("%f",ans);
}