#include <iostream>
#include <vector>
#include <iomanip>
using namespace std;
typedef long long ll;

int main(){
    string S;
    cin >> S;
    ll N = S.size();
    vector<ll> pre(26,0);
/*     for(int i=1;i<=N;i++){
        for(int j=0;j<26;j++) pre[i][j] = pre[i-1][j];
        pre[i][S[i-1]-'a'] = i;
    }
    for(int i=N;i>=1;i--){
        for(int j=0;j<26;j++) ne[i][j] = ne[i+1][j];
        ne[i][S[i-1]-'a'] = i;
    }
    ll ans = 0;
*/   
    ll ans = 0;
    for(ll i=1;i<=N;i++){
        ll l = i-pre[S[i-1]-'a'],r = N+1-i;
        ans += l*r;
        pre[S[i-1]-'a'] = i;
    }
    cout << fixed;
    cout << setprecision(10) << (long double) ans/(N*(N+1)/2) << endl;
}