#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define pb push_back
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);

    int F, N, K;
    cin >> F >> N >> K;

    double ans = 0;

    vector<double> logfact(N + 1);
    for (int i = 2; i <= N; ++i)logfact[i] = logfact[i - 1] + log(i);

    for (int x = 1; x <= F; ++x) {
        rep(a, K)for (int b = max(1, K - a); b <= N - a; ++b) {
            int c = N - a - b;

            if (a != 0 && x == F)continue;
            if (c != 0 && x == 1)continue;
            double z = logfact[N] - logfact[a] - logfact[b] - logfact[c];

            if (a > 0)z += a*log(1.0*(F-x)/F);
            z += b*log(1.0 / F);
            if (c > 0)z += c*log((x-1.0)/F);
            ans += exp(z) * x;
        }
    }

    cout << fixed << setprecision(20) << ans << endl;
}