#include<cstdio>
#include<iostream>
#include<cstring>
#define RG register
using namespace std;
long double ans,sum,n,d,x,d1,x1,sum1;
int main()
{
    cin>>n>>d>>x; sum=2*d*n+n*x*(2*n-1);
    for(int i=n;i;i--)
    {
        ans+=sum/2/n;
        d1=(2*d+2*n*d+5*x)/2/n, sum1=sum-(2*d+(2*n-1)*x)/n;
        n-=1;
        x1=(sum1-2*d1*n)/n/(2*n-1);
        d=d1, x=x1, sum=sum1;
    }
    printf("%.15lf\n",(double)ans);
    return 0;
}