#include<cstdio>
#define ld long double
int main()
{
	int n,d,x,i,j;ld ans=0,s=0,k=0;
	scanf("%d%d%d",&n,&d,&x);d=2*d+x;x*=4;
	for(i=1;i<=n;++i,ans+=s)
		if(i<2)for(s=d,k=j=1;j<=n;++j)s+=((ld)x*(i+j-2)+d)/j,k+=1./j;
		else s+=k*x+(ld)d/i-((ld)x*n+d)/(n-i+2),k+=1./i-1./(n-i+2);
	printf("%.10lf",(double)ans/4);
}