#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <chrono>
#define _USE_MATH_DEFINES
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <iterator>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;

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

const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007; // 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
/*-------------------------------------------------*/
int main() {
  cin.tie(0); ios::sync_with_stdio(false);
  // freopen("input.txt", "r", stdin);

  string s; cin >> s;
  int n = s.length();
  vector<vector<int> > same(26);
  REP(i, n) same[s[i] - 'a'].emplace_back(i + 1);
  long long bunshi = 0;
  int bunbo = 0;
  FOR(i, 1, n + 1) {
    bunshi += (long long)i * (n + 1 - i);
    bunbo += n + 1 - i;
  }
  REP(i, 26) {
    FOR(j, 1, same[i].size()) bunshi -= (long long)same[i][j - 1] * (n - same[i][j] + 1);
  }
  double ans = (double)bunshi / bunbo;
  printf("%.6f\n", ans);
  return 0;
}