//
// Created by yamunaku on 2019/07/26.
//

#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (n); i++)
#define repl(i, l, r) for(int i = (l); i < (r); i++)
#define per(i, n) for(int i = ((n)-1); i >= 0; i--)
#define perl(i, l, r) for(int i = ((r)-1); i >= (l); i--)
#define all(x) (x).begin(),(x).end()
#define MOD9 998244353
#define MOD1 1000000007
#define IINF 1000000000
#define LINF 1000000000000000000
#define SP <<" "<<
#define CYES cout<<"Yes"<<endl
#define CNO cout<<"No"<<endl
#define CFS cin.tie(0);ios::sync_with_stdio(false)
#define CST(x) cout<<fixed<<setprecision(x)

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<vector<int>> mti;
typedef vector<ll> vl;
typedef vector<vector<ll>> mtl;

int main(){
    string s;
    cin >> s;
    ld n=s.length();
    vector<queue<int>> nx(26);
    rep(i,n){
        nx[s[i]-'a'].push(i);
    }
    ld ans=0;
    rep(i,n){
        rep(j,26){
            if(!nx[j].empty()){
                ans+=n-nx[j].front();
                if(nx[j].front()==i) nx[j].pop();
            }
        }
    }
    CST(10);
    cout << ans/(n*(n+1)/2) << endl;
    return 0;
}