#include <bits/stdc++.h>
using namespace std;
inline char gc() {
  static char buf[1000000], *p1 = buf, *p2 = buf;
  return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1000000, stdin), p1 == p2)
             ? EOF
             : *p1++;
}
template <typename T>
inline T frd(T x) {
  int f = 1;
  char ch = gc();
  for (; ch < '0' || ch > '9'; ch = gc())
    if (ch == '-') f = -1;
  for (; ch >= '0' && ch <= '9'; ch = gc()) x = (x << 1) + (x << 3) + ch - '0';
  return x * f;
}
template <typename T>
inline T read(T x) {
  int f = 1;
  char ch = getchar();
  for (; ch < '0' || ch > '9'; ch = getchar())
    if (ch == '-') f = -1;
  for (; ch >= '0' && ch <= '9'; ch = getchar())
    x = (x << 1) + (x << 3) + ch - '0';
  return x * f;
}
inline void print(int x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) print(x / 10);
  putchar(x % 10 + '0');
}
const int N = 5e3, M = 26;
char s[(N << 1) + 10];
vector<int> pos[M];
int Cnt[M];
int main() {
  scanf("%s", s);
  int len = strlen(s), All = 0;
  for (int i = len; i < len << 1; i++) s[i] = s[i - len];
  for (int i = 0; i < len; i++) pos[s[i] - 'a'].push_back(i);
  for (int k = 0; k < M; k++) {
    int Max = 0;
    for (int j = 1; j < len; j++) {
      for (auto i : pos[k]) Cnt[s[i + j] - 'a']++;
      int res = 0;
      for (int i = 0; i < M; i++) res += (Cnt[i] == 1), Cnt[i] = 0;
      Max = max(Max, res);
    }
    All += Max;
  }
  printf("%.10lf\n", 1.0 * All / len);
  return 0;
}
