#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
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; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	string S;
	cin >> S;
	const int N = S.size();
	vector pos(N, vector(26, N));
	for (int i = N - 2; i >= 0; --i) {
		REP(j, 26) pos[i][j] = j == S[i + 1] - 'a' ? i + 1 : pos[i + 1][j];
	}
	
	ll ans = 0;
	REP(i, N) {
		bool done[26] {};
		priority_queue<int, vector<int>, greater<int>> pq;
		REP(j, 26) if (j != S[i] - 'a') pq.emplace(pos[i][j]);
		for (int p = i, cnt = 0; p < N;) {
			++cnt;
			done[S[p] - 'a'] = true;
			const int np = pq.top(); pq.pop();
			ans += (np - p) * cnt;
			p = np;
		}
	}
	printf("%.17f\n", (double)ans / ((ll)N * (N + 1) / 2));
}