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 bbababaaababaabbbbbabbbbbbaaabbabaaaaabbbbbaaaabbbbabaabaabababbbabbabbabaaababbabbababaaaaabaaaabbb
 cadbcdddda
 kpsaloedscghjeaqadfhmlibjepjafdomkkorinrpakondtnrnknbqarbejcenrlsbfgdbsdmkpphbkdnbitjfcofsjibssmmlll
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 abbacba
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1.000000000000000
 0.000000000000000
 0.450000000000000
 0.960000000000000
 1.000000000000000
 0.800000000000000
 1.000000000000000
 0.080000000000000
 0.840000000000000
 0.000000000000000
 0.800000000000000
 1.000000000000000
 0.000000000000000
 0.786096256684492
 1.000000000000000
 0.438502673796791

#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const long long linf = 0x3f3f3f3fLL;
const int EPS = 1e-6;
const long long mod = 1000000007;
char str[5010];
int dist[30][30];
int best[30];
int sz;
void pros(int d) {
  memset(dist, 0, sizeof dist);
  for (int i = 0; i < sz; i++) {
    int j = (i + d) % sz;
    dist[str[i] - 'a'][str[j] - 'a']++;
  }
  for (int i = 0; i < 26; i++) {
    int aux = 0;
    for (int j = 0; j < 26; j++) {
      if (dist[i][j] == 1) aux++;
    }
    best[i] = max(best[i], aux);
  }
}
int main() {
  scanf(" %s", str);
  sz = strlen(str);
  memset(best, 0, sizeof best);
  for (int i = 1; i < sz; i++) pros(i);
  int resp = 0;
  for (int i = 0; i < 26; i++) resp += best[i];
  printf("%.15lf\n", double(resp) / sz);
  return 0;
}
