#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
signed main(){
  int f,n,k;
  cin>>f>>n>>k;
  auto C=[](int n,int k){
           double res=1;
           for(Int i=0;i<k;i++){
             res*=n-i;
             res/=i+1;
           }
           return res;
         };
  auto P=[](double x,int n){
           double res=1;
           for(int i=0;i<n;i++)
             res*=x;
           return res;
         };
  auto calc=
    [&](int x)->double{
      if(x>f) return 0;
      double res=0;
      for(int y=k;y<=n;y++)
        res+=C(n,y)*P(f-x+1,n-y)*P(x-1,y);
      return res;
    };
  
  double ans=0;
  for(int x=1;x<=f;x++){
    double res=calc(x)-calc(x+1);
    ans+=x*res;
  }
  for(int i=0;i<n;i++) ans/=f;
  cout<<ans+1<<endl;
  return 0;
}