#include<bits/stdc++.h>
//ios::sync_with_stdio(false);
//cin.tie(0);
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<pii,int> ppii;
typedef pair<int,pii> pipi;
typedef pair<ll,ll> pll;
typedef pair<pll,ll> plpl;
typedef tuple<ll,ll,ll> tl;
ll mod2=1000000007;
ll mod=998244353;
ll inf=1000000000000000000;
double pi=2*acos(0);
#define rep(i,m,n) for(int i=m;i<n;i++)
#define rrep(i,n,m) for(int i=n;i>=m;i--)
int dh[4]={1,-1,0,0};
int dw[4]={0,0,1,-1};
int ddh[8]={-1,-1,-1,0,0,1,1,1};
int ddw[8]={-1,0,1,-1,1,-1,0,1};
ll lmax(ll a,ll b){
    if(a<b)return b;
    else return a;
}
ll lmin(ll a,ll b){
    if(a<b)return a;
    else return b;
}
int main(){
    string s;cin>>s;
    vector<int> v[26];
    rep(i,0,26)v[i].push_back(-1);
    rep(i,0,s.size()){
        
        v[s[i]-'a'].push_back(i);
    }
    ll sum=0;
    rep(i,0,26){
        if(v[i].size()==1)continue;
        rep(j,1,v[i].size()){
            sum+=(v[i][j]-v[i][j-1])*(s.size()-v[i][j]);
        }
        
    }
    ll u=(s.size()*(s.size()+1))/2;
    double ans=(double)sum/u;
    cout<<fixed<<setprecision(12)<<ans<<endl;
}