#include <cstdio>
#include <iostream>
using namespace std;
#define db long double
const int M = 505;
int read()
{
	int x=0,f=1;char c;
	while((c=getchar())<'0' || c>'9') {if(c=='-') f=-1;}
	while(c>='0' && c<='9') {x=(x<<3)+(x<<1)+(c^48);c=getchar();}
	return x*f;
}
int n,d,r;db cnt[M][M],sum[M][M],C[M][M];
signed main()
{
	n=read();d=read();r=read();
	C[0][0]=1;
	for(int i=1;i<=max(n,d);i++)
	{
		C[i][0]=1;
		for(int j=1;j<=i;j++)
			C[i][j]=C[i-1][j-1]+C[i-1][j];
	}
	cnt[0][0]=1;
	for(int i=1;i<=n;i++)
		for(int j=0;j<=d;j++)
			for(int k=0;k<=min(i,j);k++)
			{
				cnt[i][j]+=C[i][k]*cnt[k][j-k];
				sum[i][j]+=C[i][k]*(sum[k][j-k]
				+min(r,k)*cnt[k][j-k]);
			}
	double ans=sum[n][d]/cnt[n][d]+r;
	printf("%.7lf\n",ans);
}