#include<bits/stdc++.h>
#define Gc() getchar() 
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;using LL=__int128;
const int N=200+5,M=100+5,K=600+5,mod=1e9+7,Mod=mod-1;const db eps=1e-9;const int INF=1e9+7;mt19937 rnd(time(0));
int n;db dp[N][N],t[N][N];
void Solve(){
	int i,j;
	scanf("%d",&n);
	if(n==1){puts("0.00000000");return;}
	dp[n][0]=1;
	db ans=0;
	for(i=n;i;i--){
		for(j=(n-i)/2;~j;j--)if(j||i==n){
			db p=(i*(i-1)+2*j*(2*j-2)+2*2*j*i)*1.0/n/n;
			t[i][j]=p=1/p;
			p=i*(i-1)+2*j*(2*j-2)+2*2*j*i;
			if(i>=2) dp[i-2][j+1]+=dp[i][j]*i*(i-1)/p;
			if(j>=2) dp[i][j-1]+=dp[i][j]*2*j*(2*j-2)/p;
			if(i&&j) dp[i-1][j]+=dp[i][j]*2*2*j*i/p;
			ans+=dp[i][j]*t[i][j];
			// cerr<<i<<' '<<j<<' '<<dp[i][j]<<' '<<t[i][j]<<'\n';
		}
	}
	printf("%.8lf\n",ans+(n>2?n*n/2.0:0));
}
int main(){
	int t;
	// scanf("%d",&t);
	t=1;
	while(t--) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}