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

int main(){
	int n,d1,x;
	scanf("%d%d%d",&n,&d1,&x);
	double s=2.*n*d1+1.*x*n*(2*n-1),ans=0;
	for (; n; n--){ ans+=s/2/n; s-=s/n/n; }
	printf("%.15f\n",ans);
	return 0;
}
