#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

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

  string s;
  cin >> s;

  long long ret = 0.0;
  map<int, long long> suffix;
  REP (i, s.length()) {
    map<int, long long> tmp;
    int c = s[i] - 'a';
    for (auto &pr: suffix)
      tmp[pr.first|1<<c] += pr.second;
    tmp[1<<c]++;
    suffix = tmp;
    for (auto &pr: suffix) {
      ret += __builtin_popcountll(pr.first) * pr.second;
    }
  }
  
  long long len = ((long long)s.length() * (s.length() + 1)) / 2;
  cout << fixed << setprecision(8) << (1.0 * ret / len) << endl;
  
  return 0;
}