use std::collections::BTreeSet;


fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s = s.trim().chars().map(|c| c as usize - 'a' as usize).collect::<Vec<_>>();

    let n = s.len();
    let pattern = n * (n+1) / 2;
    let mut pos = vec![vec![]; 26];
    for i in 0..n {
        pos[s[i]].push(i);
    }
    let mut total = 0usize;
    for pos in pos.iter() {
        let mut idx = 0usize;
        for i in 0..n {
            if idx == pos.len() {
                break;
            }
            total += n - pos[idx];
            if pos[idx] == i {
                idx += 1;
            }
        }
    }
    println!("{}", total as f64 / pattern as f64);
}