#include <iostream>
#include <stdio.h>
using namespace std;
int main(int argc, char* argv[])
{
	int xa,ya,xb,yb;
	cin>>xa>>ya;
	cin>>xb>>yb;
	double x=(yb-ya);
	x/=(xb+xa);
	x*=xb;
	printf("%.10lf\n",yb-x);
	return 0;
}