#include<iostream>
#include<vector>
#include<iomanip>
using namespace std;
using ll=long long;
#define rep(i,n) for(int i=0;i<n;i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout<<fixed<<setprecision(10);
    string s;
    cin>>s;
    ll n=s.size();
    vector<vector<ll>>C(26);
    rep(i,n){
        C[s[i]-'a'].push_back(i);
    }
    double ans=0;
    rep(i,26){
        if(C[i].empty())continue;
        ll k=n*(n+1)/2-C[i].front()*(C[i].front()+1)/2-(n-1-C[i].back())*(n-C[i].back())/2;
        rep(j,(int)C[i].size()-1){
            ll l=C[i][j+1]-C[i][j];
            k-=(l)*(l-1)/2;
        }
        ans+=k/(double)(n*(n+1)/2);
    }
    cout<<ans<<"\n";
}