#include<stdio.h>
#include<stdlib.h>
#include<math.h>
int main(void){
	double a,b,c,d;
	double e;
	scanf("%lf%lf%lf%lf",&a,&b,&c,&d);
	e = fabs((d-b)/(a+c)*a+b);
	printf("%lf\n",e);
	return 0;
}