#include<bits/stdc++.h>
using namespace std;

#define REP(i,a,b) for(int i=(a),i##_end_=(b);i<i##_end_;++i)

static const int M=5003;
static const int Mod=1000000007;

int h,rrr,dw,da;
double ans,l,r;
double f(double a){
	return (dw*a*a+da*h*h-da*a*a)/(da*h-da*a+dw*a);
}
int main(){
	scanf("%d%d%d%d",&h,&rrr,&da,&dw);
	l=0,r=h*0.5;
	REP(i,0,60){
		double a=l+(r-l)/3,b=r-(r-l)/3;
		if((dw*a*a+da*h*h-da*a*a)*(da*h-da*b+dw*b)<(dw*b*b+da*h*h-da*b*b)*(da*h-da*a+dw*a))r=b;
		else l=a;
		//cout<<a<<','<<b<<','<<f(a)<<endl;
	}
	printf("%.9lf\n",(l+r)*0.5);
	return 0;
}