#include<stdio.h>

int main(){
	double a,b,x,y;
	scanf("%lf%lf%lf%lf",&a,&b,&x,&y);
	if(x/a>y/b){
		printf("%13f\n",y*(a+b)/b);
	}else {
		printf("%.13lf\n",y*(a+b)/b);
	}
	
	
	return 0;
}