#include <bits/stdc++.h>
#define FOR(i,k,n)  for(int i = (k);i < (n);++i)
#define REP(i,n)    FOR(i,0,n)
#define ALL(x)      begin(x),end(x)

using namespace std;
using vecint = vector<int>;
using ll = int64_t;

int main()
{
  string s;
  cin>>s;
  ll n = s.size();
  vecint old(26, -1);
  ll cnt = 26*n*(n+1)/2;
  REP(i,n) {
    ll d = i - old[s[i]-'a'];
    cnt -= d*(d-1)/2;
    old[s[i]-'a'] = i;
  }
  REP(i,26) {
    ll d = n - old[i];
    cnt -= d*(d-1)/2;
  }
  double div = n*(n+1)/2;
  cout<<fixed<<setprecision(10)<<(cnt/div)<<endl;
  return 0;
}