#include <iostream>
#include <stdio.h>

int main() {
	// your code goes here
	double x1,y1,x2,y2;
	scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
	if(y1==y2){
		printf("%lf\n",y1);
	}else{
		double t=x2/(x1+x2);
		printf("%lf\n",t*y1+(1-t)*y2);
	}
	return 0;
}