#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
using namespace std;

using ll = long long;
using P = pair<int, int>;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string s;
    cin >> s;
    int n = s.length();

    vector<vector<int>> pos(26, vector<int>(1, -1));
    for(int i = 0; i < n; ++i){
        pos[s[i] - 'a'].emplace_back(i);
    }

    ll cnt = 0;
    for(int i = 0; i < 26; ++i){
        if(pos[i].size() == 1)  continue;
        for(int j = 1; j < (int)pos[i].size(); ++j){
            cnt += (ll)(pos[i][j] - pos[i][j - 1]) * (n - pos[i][j]);
        }
    }

    double ans = (double)cnt / ((ll)n * (n + 1) / 2);
    cout << fixed << setprecision(15);
    cout << ans << endl;

    return 0;
}