#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
const int maxn = 500000 + 5;
const int INF = 0x3f3f3f3f;
const int P = 1000000007;
const double PI = acos(-1.0);
template <typename T>
inline T read(T& x) {
  x = 0;
  int _f = 0;
  char ch = getchar();
  while (ch < '0' || ch > '9') _f |= (ch == '-'), ch = getchar();
  while (ch >= '0' && ch <= '9') x = x * 10 + ch - '0', ch = getchar();
  return x = _f ? -x : x;
}
string s, t;
int cnt[26][5005][26], len, i, j, k;
int main() {
  cin >> s;
  t = s + s;
  len = s.length();
  for (i = 0; i < len; i++) {
    for (j = i + 1; j < i + len; j++) {
      cnt[t[i] - 'a'][j - i][t[j] - 'a']++;
    }
  }
  double ans = 0;
  for (i = 0; i < 26; i++) {
    int mx = 0;
    for (j = 1; j < len; j++) {
      int c = 0;
      for (k = 0; k < 26; k++) c += (cnt[i][j][k] == 1);
      mx = max(mx, c);
    }
    ans += 1.0 * mx / len;
  }
  printf("%.7lf\n", ans);
  return 0;
}
