#include <iostream>
#include <bits/stdc++.h>
#include <time.h>
#include <sys/timeb.h>
#include <cstdio>
#include <sys/time.h>

using namespace std;
#define ll long long
#define uint unsigned int
#define ulong unsigned long long int


class Timer{
public:
	Timer(){
		cycle_per_sec = (ulong) 2.7e9; 
	}
	void Start(){
		begin_cycle = getCycle();
	}
	double getTime(){
		return (double)(getCycle() - begin_cycle) / cycle_per_sec;
	}
	int elapsedMilliseconds(){
		return (int)( 1000 * getTime() );
	}

	ulong getCycle(){
		uint low, high;
		__asm__ volatile ("rdtsc" : "=a" (low), "=d" (high));
		return ((ulong) low) | ((ulong) high << 32);
	}
private:
	ulong begin_cycle;
	ulong cycle_per_sec;
};

Timer T;
ll Now(){return (ll) T.elapsedMilliseconds();}



template<typename T> bool InRange(T t, T l, T r){ return l <= t && t < r; }
string S;
int N;

double calc(int len){
	int dl[256]; for(int i=0;i<256;i++) dl[i] = 0;
	int dr[256]; for(int i=0;i<256;i++) dr[i] = 0;
	int lcnt = 0;
	int rcnt = 0;
	
	double cnt = 0;
	int l = 0, r = 0;
	for(int i=0;i<N;i++){
		if(l < i) l = i;
		if(r < i) r = i;
		while(l < N){
			if(dl[S[l]] > 0){
				dl[S[l]]++;
				l++;
				continue;
			}
			if(lcnt < len - 1){
				dl[S[l]] = 1;
				lcnt++;
				l++;
				continue;
			}
			break;
		}
		while(r < N){
			if(dr[S[r]] > 0){
				dr[S[r]]++;
				r++;
				continue;
			}
			if(rcnt < len){
				dr[S[r]] = 1;
				rcnt++;
				r++;
				continue;
			}
			break;
		}
		if(rcnt == len){
			cnt += r - l;
		}
		if(dl[S[i]] > 0){
			dl[S[i]]--;
			if(dl[S[i]] == 0) lcnt--;
		}
		if(dr[S[i]] > 0){
			dr[S[i]]--;
			if(dr[S[i]] == 0) rcnt--;
		}
	}
	//Console.WriteLine("len:{0}, cnt:{1}",len, cnt);
	
	double tot = (double) N * (double) (N + 1) / 2.0;
	return (double) len * cnt / tot;
	
}






int main(){
	T.Start();
	
	cin >> S;
	N = S.length();
	double ret = 0;
	for(int i=1;i<=26;i++){
		ret += calc(i);
	}
	printf("%.12f\n",ret);
	// cerr << "elapsed:" << Now() << " [ms]" << endl;
	
    return 0;
}