#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
signed main(){
  string s;
  cin>>s;
  Int n=s.size();
  map<char, Int> pos;

  double ans=0;
  for(Int i=0;i<n;i++){
    pos[s[i]]=i;

    vector<Int> vs;
    for(auto p:pos) vs.emplace_back(p.second);
    sort(vs.rbegin(),vs.rend());
    vs.emplace_back(-1);

    for(Int j=0;j+1<(Int)vs.size();j++){
      ans+=(vs[j]-vs[j+1])*(j+1);
    }
  }

  ans/=(double)n*(n+1)/2;
  cout<<ans<<endl;
  return 0;
}