#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int n;
double d,x;
int main()
{
	scanf("%d %lf %lf",&n,&x,&d);
	int m=2*n;
	double ans=0;
	while(m)
	{
		ans+=(2.0*x+(1.0*m-1.0)*d)/2.0;
		x=((1.0*m+2.0)*x+5.0*d)/(1.0*m);
		d=(1.0*m+4.0)*d/(1.0*m);
		m-=2;
	}
	printf("%.10f\n",ans);
	return 0;
}