#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using ll =  long long ;
using P = pair<int,int> ;
using pp = pair<bool,bool>;
const int INF = 1e9;
const int MOD = 1000000007;

int main(){
    string s;
    cin >> s;
    ll n = s.size();
    ll res = n*(n+1)*(n+2)/6;
    vector<vector<ll>> mp(26,vector<ll>());
    rep(i,n){
        mp[s[i]-'a'].push_back(i);
    }
    
    rep(i,26){
        int sz = mp[i].size();
        for(int j = 0;j<=sz-2;j++){
            res -= (mp[i][j]+1) * (n - mp[i][j+1]);
        }
    }
    
    double ans = (double)(res)/(double)(n*(n+1)/2);
    printf("%.7f",ans);
    return 0;
}