#include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for (int i = (int)(0); i < (int)(n); ++i)
#define REPS(i, n) for (int i = (int)(1); i <= (int)(n); ++i)
#define RREP(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define RREPS(i, n) for (int i = ((int)(n)); i > 0; i--)
#define IREP(i, m, n) for (int i = (int)(m); i < (int)(n); ++i)
#define IREPS(i, m, n) for (int i = (int)(m); i <= (int)(n); ++i)
#define FOR(e, c) for (auto &e : c)
#define SORT(v, n) sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define RVISORT(v) sort(v.begin(), v.end(), greater<int>());
#define ALL(v) v.begin(), v.end()
#define MP(n, m) make_pair(n, m);

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using ll = long long;
using ul = unsigned long;

typedef long long ll;

template<class T, class C> void chmax(T& a, C b){ a>b?:a=b; }
template<class T, class C> void chmin(T& a, C b){ a<b?:a=b; }

const int mod=1e9+7;

void printv(VI& v){
  REP(i,v.size()) cout << v[i] << " ";
  cout << endl;
}

int vtotal(VI& v){
  int total=0;
  REP(i,v.size()) total+=v[i];
  return total;
}

int main()
{
  cin.tie( 0 );
  ios::sync_with_stdio( false );

  // string n;
	// getline(cin,n);

  string s;
  cin>>s;

  int n=s.size();
  int sum=0,count=0;
  REP(i,n){
    
    map<char,int> m;
    REP(j,n-i){
      m.insert(make_pair(s[i+j],0));
      count++;
      sum+=m.size();
      // cout<<sum<<" "<<count<<endl;
    }
    // REP(j,n-i){
    //   map<char,int> m;
    //   REP(k,i+1){
    //     m.insert(make_pair(s[j+k],0));
    //   }
    //   // cout<<m.size()<<endl;
    //   sum+=m.size();
    //   count++;
    // }
  }
  // cout<<sum<<" "<<count<<endl;
  double ans=(double)sum/count;
  cout<<std::setprecision(10)<<ans<<endl;

  return 0;
}