#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 998244353;
const ll INF = mod * mod;
typedef pair<int, int> P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef pair<ll, ll> LP;
typedef vector<ll> vec;
typedef vector<string> svec;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-8;

//vector<int> loc[26];

int nex[1000000][26];

void solve() {
	string s; cin >> s;
	int n = s.length();
	/*rep(i, n) {
		loc[s[i]-'a'].push_back(i);
	}
	rep(i, 26)loc[i].push_back(n);*/
	vector<int> memo(26, n);
	per(i, n) {
		int z = s[i] - 'a';
		memo[z] = i;
		rep(j, 26) {
			nex[i][j] = memo[j];
		}
	}

	ll ans = 0;
	rep(i, n) {
		int cur = i;
		bool used[26] = {};
		used[s[i] - 'a'] = true;
		int tmp = 1;
		while (cur < n) {
			int mi = n; int chk=-1;
			rep(j, 26) {
				if (used[j])continue;
				//int id = upper_bound(loc[j].begin(), loc[j].end(), cur) - loc[j].begin();
				/*if (loc[j][id] < mi) {
					mi = loc[j][id];
					chk = j;
				}*/
				if (nex[cur][j] < mi) {
					mi = nex[cur][j]; chk = j;
				}
			}
			if (chk < 0) {
				ans += (ll)(n-cur) * tmp;
				break;
			}
			used[chk] = true;
			ll dif = mi - cur;
			ans += dif * tmp;
		   // cout << cur<<" "<<tmp << " " << dif << endl;
			cur = mi;
			tmp++;
		}
		//cout << ans << endl;
	}
	//cout << ans << endl;
	ll ss = (ll)n*(n + 1) / 2;
	cout << ans / (ld)ss << endl;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(10);
	//init();
	solve();
	//cout << "finish" << endl;
//stop
	return 0;
}