fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s = s.trim().chars().collect::<Vec<_>>();

    let n = s.len();
    let mut pos = vec![vec![]; 26];
    for i in 0..n {
        let idx = s[i] as usize - 'a' as usize;
        pos[idx].push(i);
    }
    let denom = n * (n+1) / 2;
    let mut result = 0usize;
    for i in 0..26 {
        if pos[i].is_empty() { continue; }
        let mut idx = 0usize;
        for j in 0..n {
            result += n - pos[i][idx];
            if pos[i][idx] == j {
                idx += 1;
            }
            if idx == pos[i].len() { break; }
        }
    }
    println!("{}", result as f64 / denom as f64);
}