#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    string s;
    cin >> s;
    ll n = s.size();
    vector<ll> v[26];
    rep(i,26)v[i].push_back(0);
    rep(i,n){
        v[s[i] - 'a'].push_back(i+1);
    }
    rep(i,26)v[i].push_back(n+1);

    ll x = n * (n + 1) / 2;
    ll y = n * (n + 1) / 2;

    rep(i,26){
        rep(j,v[i].size() - 1){
            x -= (v[i][j+1] - v[i][j] - 1) * (v[i][j+1] - v[i][j]) / 2;
        }
    }


    double ans = (double)x / y;
    cout << fixed << setprecision(10) << ans << endl;
    return 0;
}