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 total = n * (n + 1) / 2;
    let mut cnts = 0usize;
    for i in 0..26 {
        let mut indices = (0..n).filter(|&j| s[j] == i).map(|j| j as isize).collect::<Vec<_>>();
        indices.insert(0, -1);
        for j in 0..indices.len()-1 {
            let start = indices[j]+1;
            let end = indices[j+1];
            cnts += (end-start+1) as usize * (n - end as usize);
        }
    }
    println!("{}", cnts as f64 / total as f64);
}