#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, N);
	for (int i = N - 2; i >= 0; --i) {
		if (S[i] != S[i + 1]) pos[i] = i + 1;
		else pos[i] = pos[i + 1];
	}
	
	ll ans = 0;
	REP(i, N) {
		bool done[26] {};
		int cnt = 0;
		for (int p = i; p < N; p = pos[p]) {
			if (!done[S[p] - 'a']) {
				++cnt;
				done[S[p] - 'a'] = true;
			}
			ans += (pos[p] - p) * cnt;
		}
	}
	printf("%.17f\n", (double)ans / ((ll)N * (N + 1) / 2));
}