#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FOR(i, s, n) for (int i = (s); i < (n); i++)
#define RFOR(i, s, n) for (int i = (n) - 1; i >= (s); i--)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define ALL(a) a.begin(), a.end()
const long long MOD = 1e9+7, INF = 1e18;
template<class T>inline bool CHMAX(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T>inline bool CHMIN(T&a,T b){if(a>b){a=b;return true;}return false;}

signed main(){
	int N ;
	string s;
	cin >> s;
	N = s.size();
	vector<int>a(26,0);
	vector<int>b(26,0);
	int sum = 0,sp = 0;
	REP(i,N){
		sp += N - i;
		sum += (i+1)*(N-i);
		b[s[i]-'a'] = a[s[i]-'a'];
		a[s[i]-'a'] += i + 1;
		REP(j,26)sum -= b[j];
	}
	printf("%.12lf\n",(double)sum/sp);
}