#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> lpair;
const ll MOD = 1e9 + 7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i = (m); i < (n); i++)
#define rrep(i,m,n) for(ll i = (m); i >= (n); i--)
#define print(x) cout << (x) << endl;
#define print2(x,y) cout << (x) << " " << (y) << endl;
#define printa(x,n) for(ll i = 0; i < n; i++){ cout << (x[i]) << " \n"[i==n-1];};
ll a[1000010] = {};
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    string S;
    cin >> S;
    ll N = S.size();
    double bunbo = N * (N+1) / 2;
    memset(a, -1, sizeof(a));
    ll p2[26] = {};
    rep(i,0,26) p2[i] = -1;
    rep(i,0,N){
        ll t = S[i] - 'a';
        if(p2[t] == -1){
            a[i] = -1;
            p2[t] = i;
        }else{
            a[i] = p2[t];
            p2[t] = i;
        }
    }
    double bunshi = 0.0;
    rep(i,1,N+1){
        bunshi += i * (i+1) / 2; 
    }
    rep(i,0,N){
        if(a[i] == -1){
        }else{
            ll pos = a[i];
            ll diff_end = N - i;
            bunshi -= diff_end * (pos+1);
        }
    }
    double ans = bunshi / bunbo;
    cout << setprecision(10) << ans << endl;

}