#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;

#define fi first
#define se second
#define repl(i,a,b) for(ll i=(ll)(a);i<(ll)(b);i++)
#define rep(i,n) repl(i,0,n)
#define all(x) (x).begin(),(x).end()
#define dbg(x) cout<<#x"="<<x<<endl
#define mmax(x,y) (x>y?x:y)
#define mmin(x,y) (x<y?x:y)
#define maxch(x,y) x=mmax(x,y)
#define minch(x,y) x=mmin(x,y)
#define uni(x) x.erase(unique(all(x)),x.end())
#define exist(x,y) (find(all(x),y)!=x.end())
#define bcnt __builtin_popcountll

#define INF 1e16
#define mod 1000000007

int prv[1000010][26];
string S;

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

  cin>>S;
  int N=S.size();

  memset(prv[0],-1,sizeof(prv[0]));
  rep(i,N){
    rep(j,26){
      if(S[i]-'a'==j)prv[i+1][j]=i;
      else prv[i+1][j]=prv[i][j];
    }
  }

  double res=0;
  rep(i,N){
    ll j=max(-1,prv[i][S[i]-'a']);
    res+=(i-j)*(N-i);
  }
  printf("%.10f\n", res/(double)(N*(N+1)/2));

  return 0;
}