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#include <bits/stdc++.h>
using namespace std;
//debug
#ifdef grief
#define debug(...) do{\
	fprintf(stderr , "%s - %d : (%s) = " , __PRETTY_FUNCTION__ , __LINE__ , #__VA_ARGS__ );\
	_DO(__VA_ARGS__);\
}while(0)
template<typename I> 				void _DO(I&&x){	cerr<<x<<endl;	}
template<typename I,typename...T>	void _DO(I&&x,T&&...tail){	cerr<<x<<" , ";	_DO(tail...);	}
template<typename _a,typename _b> 	ostream& operator << (ostream &_s,const pair<_a,_b> &_p)
{	return _s<<'('<<_p.first<<','<<_p.second<<')';	}
#define IOS
#else
#define debug(...)
#define IOS do{ios_base::sync_with_stdio(0); cin.tie(0);}while(0);
#endif
//type
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef priority_queue<pll,vector<pll>,greater<pll> > heap;
//macro
#define SZ(x) ((ll)(x).size())
#define ALL(x) (x).begin(),(x).end()
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define stp setprecision(30)<<fixed
const ll INF=0x3f3f3f3f3f3f3f3f;
const ll MAX=5e3+5,Mlg=__lg(MAX)+2;
const ll MOD=1e9+7;
//}}}
string s;
vector<int> v[MAX];
int main(){
	IOS
	cin>>s;
	if(SZ(s)==1){
		cout<<1<< '\n';
		return 0;
	}
	for(int i=0;i<SZ(s);i++){
		v[s[i]].eb(i);
	}
	int cac=0,sz=SZ(s);
	for(int c='a';c<='z';c++){
		if(SZ(v[c])>26||!SZ(v[c]))	continue;
		int C[345];
		int ma=0;
		for(int i=1;i<sz;i++){
			memset(C,0,sizeof C);
			int cnt=0;
			for(int j:v[c]){
				C[s[(j+i)%sz]]++;
			}
			for(int j='a';j<='z';j++){
				if(C[j]==1)	cnt++;
			}
			ma=max(ma,cnt);
		}
		cac+=ma;
	}
	cout<<stp<<(double)cac/sz<< '\n';
	return 0;
}