#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 dp = 0, use[26];
//   int64_t sum = 0;
//   for (int i = 0;i < 26;++ i) use[i] = -1;
//   for (int i = 0;i < S.size(); ++ i) {
//     dp += i - use[S[i] - 'a'];
//     sum += dp;
//     use[S[i] - 'a'] = i;
//   }

//   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;
//     }
//   }
//   // cout<<sum<<" "<<count<<endl;
//   double ans=(double)sum/count;
//   cout<<std::setprecision(10)<<ans<<endl;

//   return 0;
// }


ll solve(string S, char c)
{
    ll las = -1;
    ll N = S.length();
    ll ret = N * (N + 1) / 2;
    S += c;
    REP(i, S.length() + 1)
    {
        if (S[i] == c)
        {
            ret -= (i - las) * (i - las - 1) / 2;
            las = i;
        }
    }
    return ret;
}

int main()
{
    string S;
    cin >> S;
    ll su = 0, N = S.length();
    REP(c, 26)
    {
        su += solve(S, (char)('a' + c));
    }
    cout << (double)su / (N * (N + 1) / 2);
}