#include "bits/stdc++.h"
using namespace std;
#define Rep(i,n) for(int i=0;i<(int)(n);i++)
#define For(i,n1,n2) for(int i=(int)(n1);i<(int)(n2);i++)
#define REP(i,n) for(ll i=0;i<(ll)(n);i++)
#define RREP(i,n) for(ll i=((ll)(n)-1);i>=0;i--)
#define FOR(i,n1,n2) for(ll i=(ll)(n1);i<(ll)(n2);i++)
#define RFOR(i,n1,n2) for(ll i=((ll)(n1)-1);i>=(ll)(n2);i--)
#define put(a) cout<<a<<"\n"
#define all(a)  (a).begin(),(a).end()
#define SORT(a) sort((a).begin(),(a).end())
#define oorret 0
#define oor(x) [&](){try{x;} catch(const out_of_range& oor){return oorret;} return x;}()
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
template<typename T1, typename T2> inline bool chmin(T1& a, T2 b) { if (a > b) { a = b; return 1; }return 0; }
template<typename T1, typename T2> inline bool chmax(T1& a, T2 b) { if (a < b) { a = b; return 1; }return 0; }

vector<vector<int> > calcNext(const string& S) {
	int n = (int)S.size();
	vector<vector<int> > res(n + 1, vector<int>(26, n));
	for (int i = n - 1; i >= 0; --i) {
		for (int j = 0; j < 26; ++j) res[i][j] = res[i + 1][j];
		res[i][S[i] - 'a'] = i;
	}
	return res;
}

int main() {
	string s;
	cin >> s;
	vector<vector<int>> cn = calcNext(s);
	ll n = s.size();
	ll a = n*(n+1)/2;
	//ll b = 0;
	double res = 0;
	REP(i, n) {
		ll b = 0;
		REP(j, 26) {
			if (cn[i][j] != n) {
				b += n - cn[i][j];
			}
		}
		res += (double)b / a;
	}
	cout << setprecision(20) << res << endl;
 	return 0;
}