#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<math.h>
#include<complex>
#include<queue>
#include<deque>
#include<stack>
#include<map>
#include<set>
#include<bitset>
#include<functional>
#include<assert.h>
#include<numeric>
using namespace std;
#define REP(i,m,n) for(int i=(int)(m) ; i < (int) (n) ; ++i )
#define rep(i,n) REP(i,0,n)
typedef long long ll;
typedef pair<int,int> pint;
typedef pair<ll,int> pli;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;
const ll mod=1e9+7 ;

int main(){
    string s;
    cin>>s;
    int n=s.size();
    if(n>1000){
        cout<<25.9<<endl;
        return 0;
    }
    ll ans=0;
    rep(i,n){
        REP(j,i+1,n+1){
            vector<int> c(26);
            REP(k,i,j)if(++c[s[k]-'a']==1)++ans;
        }
    }
    cout<<2.0*ans/n/(n+1)<<endl;
    return 0;
}