#pragma region
#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std; typedef long double ld; typedef long long ll;
typedef unsigned long long ull;
#define endl "\n"
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define rep(i,n) for(int i=0;i<(n);i++)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define ALL(x) (x).begin(), (x).end()
constexpr int INF=1<<30; constexpr ll LINF=1LL<<60; constexpr ll mod=1e9+7; constexpr int NIL = -1;
template<class T>inline bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
#pragma endregion
//-------------------
map<char, vector<int>> mp;


int main(){
    cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(15);
    string s; cin >> s;
    int n = s.size();
    
    auto f = [](ll n) { return (n+1)*n/2; };
    rep(i,26){
        mp[(char)(i+(int)'a')].emplace_back(-1);
    }
    rep(i,n) {
        mp[s[i]].emplace_back(i);
    }
    rep(i,26){
        mp[(char)(i+(int)'a')].emplace_back(n);
    }
    ll ans = f(n) * 26;
    rep(i, 26) {
        auto v = mp[(char)(i+(int)'a')];
        rep(i, v.size()-1) {
            int diff = v[i+1] - v[i];
            ans -= f(diff-1);
        }
    }

    ld res = ans;
    cout << res / f(n) << endl;
    return 0;
}