#include<bits/stdc++.h>

using namespace std;

int n;
long double d,x,ans;

int main()
{
	cin>>n>>d>>x;
	for(int i=n;i;i--)
	{
		ans+=d+(i*2-1)*x/2;
		d=(d*(i*2+2)+x*5)/(i*2);
		x=(i*2+4)*x/(i*2);
	}
	printf("%.10lf\n",(double)ans); 
} 