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

//template
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(a);i>(b);i--)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll; typedef pair<ll, ll> P;
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; }
template<typename A,size_t N,typename T>void Fill(A(&array)[N],const T &val){fill((T*)array, (T*)(array+N), val);}
const int inf = INT_MAX / 2; const ll INF = LLONG_MAX / 2;
//template end



int main(){
    string s; cin>>s; int n=s.size();
    ll ans=0; int dp=0,used[26]; Fill(used,-1);
    rep(i,0,n){
        dp+=i-used[s[i]-'a']; ans+=dp; used[s[i]-'a']=i;
    }
    printf("%.12f\n",(double)(ans*1.0/(1LL*n*(n+1)/2)));
    return 0;
}