#include <cstring>
#include <cmath>
#include <cstdio>
#include <cassert>
#include <climits>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <deque>
#include <utility>
#include <algorithm>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <bitset>
#include <functional>
#include <numeric>
#include <iomanip>
#include <iterator>
#include <array>
using namespace std;
#define rt return
#define FOR(i,j,k) for(int i=j; i<(int)k;++i)
#define rep(i,j) for(int i=0;i<(int)j;++i)
#define rep1(i,j) for(int i=1;i<=(int)j;++i)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define pb_ push_back
#define eb_ emplace_back
#define fi_ first
#define se_ second
#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 p_ printf
#define s_ scanf
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

vector<vector<int>> combinations(int n){
    auto res = vector<vector<int>>(n + 1, vector<int>(n + 1));
    rep(i, n + 1) res[i][0] = 1;
    for(int i = 1; i <= n; ++i) for(int j = 1; j <= i; ++j)
        res[i][j] = (res[i - 1][j - 1] + res[i - 1][j]);
    return res;
}

vector<vi> C;
int N;

void solve(){
    C = combinations(N);
    double ans = 1.0, p;
    for(int i = 2; i <= N; ++i){ // card i
        // はさまれる
        FOR(j, 1, i)FOR(k, j + 1, i){
            p = (i - 2.0) / i;
            p *= 1.0 / C[i-1][2];
            ans += p*j*k;
        }
        // 端
        p = 2.0 / i;
        ans += p*1.0;
    }
    printf("%0.15f\n", ans);
}

int main(){
    while(cin >> N){
        solve();
    }
}