#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> p_ll;

template<class T>
void debug(T itr1, T itr2) { auto now = itr1; while(now<itr2) { cout << *now << " "; now++; } cout << endl; }
#define repr(i,from,to) for (ll i=(ll)from; i<(ll)to; i++)
#define all(vec) vec.begin(), vec.end()
#define rep(i,N) repr(i,0,N)
#define per(i,N) for (int i=(int)N-1; i>=0; i--)

const ll MOD = pow(10,9)+7;
const ll LLINF = pow(2,61)-1;
const int INF = pow(2,30)-1;

vector<ll> fac;
void c_fac(int x=pow(10,6)+10) { fac.resize(x,true); rep(i,x) fac[i] = i ? (fac[i-1]*i)%MOD : 1; }
ll inv(ll a, ll m=MOD) { ll b = m, x = 1, y = 0; while (b!=0) { int d = a/b; a -= b*d; swap(a,b); x -= y*d; swap(x,y); } return (x+m)%m; }
ll nck(ll n, ll k) { return fac[n]*inv(fac[k]*fac[n-k]%MOD)%MOD; }
ll gcd(ll a, ll b) { if (a<b) swap(a,b); return b==0 ? a : gcd(b, a%b); }
ll lcm(ll a, ll b) { return a/gcd(a,b)*b; }


int main() {
  string S; cin >> S;
  ll l = S.size();
  vector<ll> pos[26] = {}; rep(i,26) pos[i].push_back(-1);
  rep(i,l) pos[(int)(S[i]-'a')].push_back(i);
  rep(i,26) pos[i].push_back(l);
  // rep(i,26) debug(all(pos[i]));

  double count = 0;
  rep(i,26) rep(j,pos[i].size()-1) {
    double diff = pos[i][j+1]-pos[i][j];
    count += diff*(diff-1)/2;
  }
  // cout << count << endl;

  double result = 26 - count/((l+1)*l/2);
  cout << setprecision(10) << result << endl;
  return 0;
}