#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cstdlib>
#include <queue>
using namespace std;
typedef double f2;
int n;f2 d,x;
int main() {
	scanf("%d%lf%lf",&n,&d,&x);
	int i; f2 ans=0;
	for(i=n;i;i--) {
		f2 nd=d+(2*d+5*x)/(2*i);
		f2 nx=d+x+(2*d+9*x)/(2*i)-nd;
		ans+=d+x*(2*i-1)/2;
		d=nd, x=nx;
	}
	printf("%.12f\n",ans);
}
