#include<bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl

ll f(ll n){
    return n * (n+1) / 2;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    string s; cin >> s;
    ll L = s.size();

    double ans = 0;
    FOR(i, 0, 26){
        char c = 'a'+i;
        ll sum = f(L);
        ll prev = -1;
        FOR(j, 0, L){
            if(s[j]==c){
                sum -= f(j - prev - 1);
                prev = j;
            }
        }
        sum -= f(L - prev - 1);
        ans += sum;
    }

    ans /= f(L);
    cout <<
     setprecision(20);
    p(ans);
    
    return 0;
}