#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <random>
#include <chrono>
#include <queue>
#include <ctime>
using namespace std;
typedef long long ll;
typedef pair<int, int> PII;
typedef pair<ll, ll> PLL;
#define fs first
#define sc second
#define INF 1000000000
#define MOD 1000000007
#define EPS 0.00000001


int main() {
    string S; cin >> S;
    int N = (int)S.size();
    vector<ll> dp(N+1);
    ll sum = 0;
    vector<int> use(26, -1);
    for(int i=0; i<N; i++){
        dp[i+1] += dp[i] + i - use[S[i] - 'a'];
        sum += dp[i+1];
        use[S[i] - 'a'] = i;
    }
    cout << fixed << setprecision(10) << (double)sum/((ll)N*(N+1)/2) << endl;
    return 0;
}