#include <iostream>
#include <cstring>
#include <ctime>
#include <algorithm>
#include <cmath>
#include <vector>
#include <queue>
#include <set>
#include <map>
using namespace std;
typedef long long int ll;
const int N=510,INF=1e9;
const double eps=1e-5;
typedef pair <int,int> PII;
inline int read()
{
    int x=0,f=1;char c=getchar();
    while(c<'0' || c>'9') {if(c=='-') f=-1;c=getchar();}
    while(c>='0' && c<='9') {x=x*10+c-48,c=getchar();}
    return x*f;
}
inline ll readll()
{
    ll x=0,f=1;char c=getchar();
    while(c<'0' || c>'9') {if(c=='-') f=-1;c=getchar();}
    while(c>='0' && c<='9') {x=x*10+c-48,c=getchar();}
    return x*f;
}
int n,d,r;
double f[N][N],g[N][N],C[N][N],a,b;
int main()
{
    // #define FILEIO
    #ifdef FILEIO
        freopen("in.in","r",stdin);
        freopen("out.out","w",stdout);
    #endif
    n=read(),d=read(),r=read();
    C[0][0]=1;
    for(int i=1;i<=n;i++)
    {
        C[i][0]=1;
        for(int j=1;j<=i;j++) C[i][j]=C[i-1][j-1]+C[i-1][j];
    }
    f[n][0]=1;
    for(int s=0;s<d;s++)
    {
        for(int i=1;i<=n;i++)
        {
            for(int j=1;j<=i && s+j<=d;j++)
            {
                f[j][s+j]+=f[i][s]*C[i][j];
                g[j][s+j]+=(g[i][s]+f[i][s]*min(r,j))*C[i][j];
            }
        }
    }
    for(int i=1;i<=n;i++) a+=f[i][d],b+=g[i][d];
    printf("%.7lf",b/a+r);
    cerr<<endl<<1e3*clock()/CLOCKS_PER_SEC<<"ms";
    return 0;
}