#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

//ret[i][j] : i以降の文字jが登場する最初のindex
vector<vector<int>> fillnxt(string s){
    int n=(int)s.size();
    vector<vector<int>> ret(n+1,vector<int>(26,INF));
    for(int i=n-1;i>=0;i--){
        rep(j,26) ret[i][j]=ret[i+1][j];
        ret[i][s[i]-'a']=i;
    }
    return ret;
}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout<<fixed<<setprecision(8);
    
    string s;cin>>s;
    int n=(int)s.size();
    auto nxt=fillnxt(s);

    ll sum=0;
    rep(j,26){
        rep(i,n){
            if(nxt[i][j]==INF) continue;
            sum+=n-nxt[i][j];
        }
    }
    ll bo=(ll)(1ll*n*(n+1)/2);
    cout<<((double)sum)/((double)bo)<<endl;
    return 0;
}