???

#include<bits/stdc++.h>
using namespace std;
//(x*a-y*b)/(a-b)
int main(){
	int x,a,y,b;
	cin>>x>>a>>y>>b;
	float i;
	i=(x*a-y*b)/(a-b);
	printf("%.2lf\n",i);
	return 0;
}