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 used_at = vec![vec![]; 26];
    for i in 0..n {
        used_at[s[i]].push(i);
    }
    for i in 0..26 {
        used_at[i].push(n);
    }
    let mut result = 0usize;
    for i in 0..26 {
        for j in 0..used_at[i].len() {
            let left = if j > 0 { used_at[i][j-1]+1 } else { 0 };
            let right = used_at[i][j];
            result += (n - right) * (right - left + 1);
        }
    }
    println!("{}", result as f64 / total_pattern as f64);
}