#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using ll =  long long ;
using P = pair<int,int> ;
const int INF = 1e9;
const int MOD = 1000000007;

int main(){
    string s;
    cin >> s;
    ll n = s.size();
    vector<int> use(26,-1);
    vector<ll> dp(n,0);
    ll res = 0;
    rep(i,n){
        if(i==0){
            dp[i] = 1;
            use[s[i]-'a'] = i;
        }else{
            dp[i] = dp[i-1] + i - use[s[i]-'a'];
            use[s[i]-'a'] = i;
        }
        res += dp[i];
    }
    double ans = (double)(res)/(double)(n*(n+1)/2);
    printf("%.7f",ans);
    return 0;
}