#include <bits/stdc++.h>
using namespace std;
using lint = long long;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;

struct DisjointSparseTable {
  using T = int;
  static T op(const T& x, const T& y) { return x | y; }

  const int n;
  VV<T> t;
  template<class Itr> DisjointSparseTable(Itr first, Itr last) :
    n(distance(first, last)), t(__lg(n) + 1, V<T>(first, last)) {
    for (int k = 1; k < (int) t.size(); ++k) {
      for (int i0 = 1 << k; i0 < n; i0 += 1 << k + 1) {
        for (int i = i0 + 1; i < min(i0 + (1 << k), n); ++i) {
          t[k][i] = op(t[k][i - 1], t[k][i]);
        }
        for (int i = i0 - 2; i >= i0 - (1 << k); --i) {
          t[k][i] = op(t[k][i], t[k][i + 1]);
        }
      }
    }
  }
  T acc(int l, int r) const {
    assert(l < r);
    if (l == --r) return t[0][l];
    int k = __lg(l ^ r);
    return op(t[k][l], t[k][r]);
  }
};

int main() {
  cin.tie(nullptr); ios::sync_with_stdio(false);
  string s; cin >> s;
  int n = s.size();
  V<> a(n);
  for (int i = 0; i < n; ++i) {
    a[i] = 1 << s[i] - 'a';
  }
  DisjointSparseTable dst(begin(a), end(a));
  VV<> b(n, V<>(27));
  for (int k = 1; k <= 26; ++k) {
    int r = 0;
    for (int l = 0; l < n; ++l) {
      while (r < n and __builtin_popcount(dst.acc(l, r + 1)) <= k) ++r;
      b[l][k] = r;
    }
  }
  lint sum = 0;
  for (int l = 0; l < n; ++l) {
    b[l][0] = l;
    for (int k = 1; k <= 26; ++k) {
      sum += (b[l][k] - b[l][k - 1]) * k;
    }
  }
  cout << fixed << setprecision(15) << sum / (0.5 * n * (n + 1)) << '\n';
}