fabbbhgedd
 abbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaababbbaab
 hcdhgcchbdhbeagdcfedgcbaffebgcbcccadeefacbhefgeadfgchabgeebegahfgegahbddedfhffeadcedadgfbeebhgfahhfb
 khjcoijiicdkdianmdolmadobdkcmgifdnffddnjehhbldlkjffknficdcmokfacioiegjedbmadjioomdacbodcajcmonmnlabo
 bababbdaee
 cbbbbcaaca
 gaejllebhn
 eaaebccaeacdecaedcaabbbdeebccdcdaabeeaeeaddbaabdccebecebbbbedbdcbbbbbbecbaddcddcccdcbbadbecddecedbba
 difhjdjbcdjedhiegagdejkbjfcdcdagdijdjajecbheiabfbjdgjdecfhdkgdbkcgcgakkiiggfkgcfadkjhiijkjacgejfhjge
 bbababaaababaabbbbbabbbbbbaaabbabaaaaabbbbbaaaabbbbabaabaabababbbabbabbabaaababbabbababaaaaabaaaabbb
 cadbcdddda
 kpsaloedscghjeaqadfhmlibjepjafdomkkorinrpakondtnrnknbqarbejcenrlsbfgdbsdmkpphbkdnbitjfcofsjibssmmlll
 aaabbbaaaabbbbaaabbbbbaabbbbaaababbaaabbbbaaabbbbababbbbaaabbbbaaabbbbbaabbbbaaabbbbaaabbbb
 ibledofnibedebifmnjdoaijeghajecbkjaebbkofnacceaodiifbhgkihkibddneeiemacodeafeaiiiaoajhmkjffbmmiehebhokfklhbkeoanoajdedjdlkbhenidclagggfhhhldfleccgmjbkhaginlhabkabagikalccndciokabfaebjkndf
 abbacba
 jkeaagakbifeaechkifkdghcjcgighidcgdccfbdbcackfgaebkddabgijkhjkaffkabacekdkjekeccegbecbkecbgbgcacgdackcdfjefaifgbigahkbedidfhjbikejdhejcgideaeejdcegeeccaefbddejkbdkfagfcdjbikbidfggkidcdcic

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>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define F first
#define S second
#define FBO find_by_order
#define OOK order_of_key
#define mset(m,v) memset((m),(v),sizeof(m))
#define inrep int TC;cin>>TC;while(TC--)
#define cppio ios::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define PI 3.1415926535897932384626433832795028841971693993751
#define TRACE
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1<<" | ";
  __f(comma+1, args...);
}
#else
#define trace(...)
#endif

typedef long long ll;
typedef long double ld;
typedef vector <int> vi;
typedef vector <ll> vll;
typedef pair <int,int> pii;
typedef pair <ll,ll> pll;
typedef vector <pii> vpii;
typedef vector <pll> vpll;
typedef tree <int, null_type, less <int>, rb_tree_tag, tree_order_statistics_node_update> pbds;

// SOLUTION

string s;
int dp[26][5005][26];

int main () {
	cppio;
    cin >> s;
    int n = s.size();
    string k = s + s;
    for (int i = 0; i < n; i++) {
        for (int j = 1; j < n; j++) {
            dp[k[i] - 'a'][j][k[i + j] - 'a']++;
        }
    }
    vi mvs(n);
    for (int i = 0; i < 26; i++) {
        for (int j = 0; j < n; j++) {
            for (int k = 0; k < 26; k++) {
                if (dp[i][j][k] == 1) mvs[j]++;
            }
        }
    }
    ld num = *max_element (mvs.begin(), mvs.end());
    cout << fixed << setprecision(15) <<  (num / n) << "\n";
	return 0;
}
