#include<bits/stdc++.h>
using namespace std;
double work(int n,double d,double x)
{//cout<<n<<' '<<d<<' '<<x<<endl;
	if (!n) return 0;
	return work(n-1,d+(double)d/n+2.5*x/n,(double)(n+2)/n*x)+d+(double)(n+n-1)*x/2;
}
int main()
{
	int n;double d,x;scanf("%d%lf%lf",&n,&d,&x);
	printf("%.10lf\n",work(n,d,x));
}