#include <bits/stdc++.h>
using namespace std;
#define MOD 1000000007
//#define MOD 998244353
#define INF 1145141919810893364
typedef pair<int,int> PP;
typedef long long ll;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
#define int ll
#define setdouble setprecision
#define REP(i,n) for(int i=0;i<(n);++i)
#define OREP(i,n) for(int i=1;i<=(n);++i)
#define RREP(i,n) for(int i=(n)-1;i>=0;--i)
#define GOODBYE cout << -1 << endl;return 0
#define MM <<" "<<
#define Endl endl



signed main(void){
    string S;
    cin >> S;
    vector<int> K[30]={};
    int N=S.size();
    double Ans=0;
    int B=0;
    
    REP(i,N){
        K[(int)(S[i]-'a')].push_back(i);
    }
    
    REP(i,N){
        B+=(i+1)*(N-i);
    }
    
    REP(i,26){
        REP(j,((int)K[i].size())-1){
            int l=K[i][j],r=K[i][j+1];
            B-=(l+1)*(N-r);
        }
    }
    Ans=B;
    //cout << Ans << endl;
    Ans/=N;
    Ans*=2;
    Ans/=N+1;
    printf("%.8f\n",Ans);
    return 0;
}