#include <algorithm> #include <bitset> #include <cassert> #include <cctype> #include <chrono> #define _USE_MATH_DEFINES #include <cmath> #include <cstdio> #include <cstring> #include <ctime> #include <deque> #include <functional> #include <iostream> #include <iterator> #include <map> #include <queue> #include <random> #include <set> #include <sstream> #include <string> #include <tuple> #include <utility> #include <vector> using namespace std; #define FOR(i,m,n) for(int i=(m);i<(n);++i) #define REP(i,n) FOR(i,0,n) #define ALL(v) (v).begin(),(v).end() const int INF = 0x3f3f3f3f; const long long LINF = 0x3f3f3f3f3f3f3f3fLL; const double EPS = 1e-8; const int MOD = 1000000007; // 998244353; const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1}; /*-------------------------------------------------*/ int main() { cin.tie(0); ios::sync_with_stdio(false); // freopen("input.txt", "r", stdin); string s; cin >> s; int n = s.length(); vector<vector<int> > same(26); REP(i, n) same[s[i] - 'a'].emplace_back(i + 1); long long bunshi = 0, bunbo = 0; FOR(i, 1, n + 1) { bunshi += (long long)i * (n + 1 - i); bunbo += n + 1 - i; } REP(i, 26) { FOR(j, 1, same[i].size()) bunshi -= (long long)same[i][j - 1] * (n - same[i][j] + 1); } double ans = (double)bunshi / bunbo; printf("%.6f\n", ans); return 0; }