use proconio::{input, marker::Bytes};

fn main() {
    input! {
        s:Bytes,
    }

    let s = s.into_iter().map(|si| si - b'a').collect::<Vec<_>>();
    const C: usize = 26;
    let n = s.len();
    let mut poss = vec![vec![0; C]; n + 1];
    for i in 0..n {
        for j in 0..C {
            if s[i] == j as u8 {
                poss[i + 1][j] = i + 1;
            } else {
                poss[i + 1][j] = poss[i][j];
            }
        }
    }

    let mut ans = 0;
    for i in 1..=n {
        let mut poss_cp = poss[i].clone();
        poss_cp.sort();

        let mut cnts = vec![0; C + 1];
        let mut prev = 0;
        for j in (1..=C).rev() {
            cnts[j] = poss_cp[C - j] - prev;
            prev = poss_cp[C - j];
        }

        // println!("i:{} {:?}",i,cnts);
        for j in 1..=C {
            ans += j * cnts[j];
        }
    }

    let ans = ans as f64 / (n * (n + 1) / 2) as f64;
    println!("{}", ans);
}