#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();
	ll ans = 0;
	int prv[26];
	memset(prv, -1, sizeof(prv));
	REP(i, N) ans += (ll)(i - prv[S[i] - 'a']) * (N - i), prv[S[i] - 'a'] = i;
	printf("%.17f\n", (double)ans / ((ll)N * (N + 1) / 2));
}