#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	string s; cin >> s;
	long long sz = s.size();
	vector pos(26, vector<int>(1, 0));
	rep(i, s.size()) {
		pos[s[i] - 'a'].push_back(i + 1);
	}
	rep(i, 26)pos[i].push_back(sz + 1);
	double ans = 0;
	long long all = (sz + 1)*sz / 2;
	rep(i, 26) {
		long long val = all;
		rep(j, pos[i].size() - 1) {
			long long x = pos[i][j + 1] - pos[i][j];
			val -= x * (x - 1) / 2;
		}
		ans += val;
	}
	ans /= ((double)sz + 1.)*sz / 2.;
	printf("%.16f\n", ans);
	return 0;
}