fn lower_bound(val: usize, target: &Vec<usize>) -> usize {
    let mut lower = 0usize;
    let mut upper = target.len();
    while upper > lower {
        let middle = (upper + lower) / 2;
        if val <= target[middle] {
            upper = middle;
        } else {
            lower = middle+1;
        }
    }
    upper
}

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_pattern = n * (n + 1) / 2;
    let mut cnt = 0usize;
    let mut patterns = vec![vec![]; 26];
    for i in 0..n {
        patterns[s[i]].push(i);
    }
    for i in 0..26 {
        for j in 0..n {
            if s[j] == i {
                cnt += n - j;
                continue;
            }
            let lidx = lower_bound(j, &patterns[i]);
            if lidx == patterns[i].len() { continue; }
            let lidx = patterns[i][lidx];
            cnt += n - lidx;
        }
    }
    println!("{}", cnt as f64 / total_pattern as f64);
}