#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

void solve() {
    string S;
    cin >> S;
    int N = sz(S);
    const int R = 26;
    vi id[R];
    rep(i, R)id[i].push_back(-1);
    rep(i, N) {
        assert('a' <= S[i] && S[i] <= 'z');
        id[S[i] - 'a'].push_back(i);
    }

    ll sm = 0;
    rep(i, R) {
        for (int j = 1; j < sz(id[i]); ++j) {
            sm += (ll)(id[i][j] - id[i][j - 1])*(N - id[i][j]);
        }
    }
    ll tot = (ll)N*(N + 1) / 2;
    double ans = (double)sm / tot;
    cout << ans << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	solve();
	return 0;
}