#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
    int f, n, k;
    cin>>f>>n>>k;
    double p[110];
    p[f+1]=0;
    for(int i=1; i<=f; i++){
        p[i]=0;
        double c=1;
        for(int j=1; j<k; j++){
            c*=(double)(n-j+1);
            c/=(double)j;
        }
        for(int j=k; j<=n; j++){
            c*=(double)(n-j+1);
            c/=(double)j;
            double a=c;
            for(int l=0; l<j; l++){
                a*=(double)(f-i+1);
                a/=(double)f;
            }
            for(int l=0; l<n-j; l++){
                a*=(double)(i-1);
                a/=(double)f;
            }
            p[i]+=a;
        }
    }
    double ans=0;
    for(int i=1; i<=f; i++){
        ans+=(double)i*(p[i]-p[i+1]);
    }
    printf("%.7lf\n", ans);
    return 0;
}