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

int read(){
	int x=0,f=1;char ch=getchar();
	for (;!isdigit(ch);ch=getchar()) if (ch=='-')  f=-f;
	for (;isdigit(ch);ch=getchar()) x=x*10+ch-'0';
	return x*f;
}

int main(){
	double n=read(),d=read(),x=read(),ans=0;
	while (n){
		ans+=d+x*(2*n-1)/2;
		d=((2*n+2)*d+5*x)/(2*n);
		x*=(n+2.0)/n;
		n--; 
	}
	printf("%.12lf\n",ans);
	return 0;
}