#include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <vector>
#include <cmath>
#include <cstdlib>
#include <queue>
#include <stack>
#include <iomanip>


using namespace std;

#define REP(i, n) for(ll i = 0;i < n;i++)
#define REPR(i, n) for(ll i = n;i >= 0;i--)
#define FOR(i, m, n) for(ll i = m;i < n;i++)
#define FORR(i, m, n) for(ll i = m;i >= n;i--)
#define REPO(i, n) for(ll i = 1;i <= n;i++)
#define ll long long
#define INF (ll)1 << 60
#define MINF (-1 * INF)
#define ALL(n) n.begin(),n.end()
#define MOD (ll)1000000007
#define P pair<ll, ll>


string s;
vector<ll> cnt[26];
double ans;
int main() {
	cin >> s;
	REP(i, 26) {
		cnt[i].push_back(-1);
		REP(j, s.size()) {
			if (s[j] - 'a' == i)cnt[i].push_back(j);
		}
		cnt[i].push_back(s.size());
	}
	REP(i, 26) {
		double now = 0;
		REP(j, cnt[i].size() - 1) {
			ll n = cnt[i][j + 1] - cnt[i][j];
			now += n * (n - 1) / 2;
		}
		double sz = ((s.size()) *(s.size() + 1) / 2);
		ans += (sz - now) / sz;
 	}
	cout << ans << endl;
}