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

#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define test cout<<"test"<<endl;
#define fi first
#define se second
#define rep(i,n,m) for(ll i = n; i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = n; i < (ll)(m); i--)
typedef long long ll;
typedef long double ld;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
void chmin(ll &a,ll b){if(a>b)a=b;}
void chmax(ll &a,ll b){if(a<b)a=b;}
void pmod(ll &a,ll b){a=(a+b)%MOD;}
void pmod(ll &a,ll b,ll c){a=(b+c)%MOD;}
void qmod(ll &a,ll b){a=(a*b)%MOD;}
void qmod(ll &a,ll b,ll c){a=(b*c)%MOD;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
void ans(bool x,ll y,ll z){if(x)cout<<y<<endl;else cout<<z<<endl;}
void ans(bool x,string y,string z){if(x)cout<<y<<endl;else cout<<z<<endl;}   
void debug(vector<vector<ll>>v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
void debug(vector<ll>v,ll n){cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
ll gcd(ll x,ll y){ll r;while((r=x%y)!=0){x=y;y=r;}return y;}
//m.emplace(x,0).fi->second++;

int main(){
  ll res=0,res1=INF,res2=-INF,buf=0;
  bool judge = true;
  string s;cin>>s;
  ll n=s.size();
  res=n*(n+1)/2*26;
  rep(i,0,26){
    buf=-1;
    char bufc='a'+i;
    rep(j,0,n){
      if(s[j]==bufc){
        ll cnt=j-buf-1;
        res-=(cnt)*(cnt+1)/2;
        buf=j;
      }
    }
    ll cnt=n-buf-1;
    res-=(cnt)*(cnt+1)/2;
  }
  ld x=n;
  cout<<setprecision(10)<<fixed<<res/x/(x+1)*2<<endl;

  return 0;
}