#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll mod=998244353;
ll inf=100990999999999909;
int main(){
    string s;
    cin >> s;
    vector<ll>memo(100,-1);
    ll ans=0;
    for (ll i = 0; i < s.size(); i++)
    {
        map<ll,ll>mm;
        ll now=0;
        memo[s[i]-'a']=max(memo[s[i]-'a'],i);
        for (char c = 'a'; c <='z'; c++)
        {
            if (memo[c-'a']!=-1)
            {
                now++;
                mm[memo[c-'a']]++;
            }
        }
        ll pre=-1;
        for(auto v:mm){
            ans+=now*(v.first-pre);
            pre=v.first;
            now--;
        }

    }
    ll n=s.size();
    ll x=(n*(n+1))/2;
    ld y=ans;
    ld z=x;
    cout << setprecision(100) << y/z << endl; 
}