#include<bits/stdc++.h>
using namespace std;
#define inf 1000000000
#define INF 1000000000000000
#define ll long long
#define ull unsigned long long
#define M (int)(1e9+7)
#define P pair<int,int>
#define PLL pair<ll,ll>
#define FOR(i,m,n) for(int i=(int)m;i<(int)n;i++)
#define RFOR(i,m,n) for(int i=(int)m;i>=(int)n;i--)
#define rep(i,n) FOR(i,0,n)
#define rrep(i,n) RFOR(i,n,0)
#define all(a) a.begin(),a.end()
#define IN(a,n) rep(i,n){ cin>>a[i]; }
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define int ll
#define vi vector<int>



signed main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  string s;
  cin>>s;

  vector<int> a(30,0);
  vector<int> pre(30,-1);

  rep(i,s.size()){
    int c=s[i]-'a';
    vector<int> p(30);
    rep(j,30){
      p[j]=pre[j];
    }
    int k=1;
    int t=-1;
    int e=-1,r=i;
    while(true){
      rep(j,30){
        if(j==c) continue;
        if(t<p[j]){
          t=p[j];
          e=j;
        }
      }
      if(t==-1){
        a[k]+=r+1;
        break;
      }else{
        a[k]+=r-t;
        p[e]=-1;
        r=t;
        k++;
        t=-1;
      }
    }
    pre[c]=i;
  }

  double sum=0;
  rep(i,30){
    sum+=i*a[i];
    //cout<<a[i]<<endl;
  }

  double n=s.size();

  printf("%.10lf\n",sum/(n*(n+1)/2));



}