use proconio::input;

fn main() {
    input! {
        (n, k): (usize, u64),
        t: String,
    }

    let encrypt = t
        .chars()
        .map(|ch| ch.to_digit(10).unwrap() as u64)
        .collect::<Vec<_>>();
    let mut suffix_num_combs = vec![0_u64; n + 2];
    suffix_num_combs[n] = 1;
    for i in (0..n).rev() {
        let num_combs_1 = suffix_num_combs[i + 1] * valid_encryption(&encrypt[i..i + 1]) as u64;
        let num_combs_2 =
            suffix_num_combs[i + 2] * (i + 1 < n && valid_encryption(&encrypt[i..i + 2])) as u64;
        suffix_num_combs[i] = num_combs_1.saturating_add(num_combs_2);
    }

    let mut plaintext = String::new();
    let mut overcome = 0_u64;
    let mut progress = 0_usize;
    while progress < n {
        if valid_encryption(&encrypt[progress..progress + 1])
            && overcome + suffix_num_combs[progress + 1] >= k
        {
            plaintext.push((b'a' + encrypt[progress] as u8 - 1) as char);
            progress += 1;
        } else {
            plaintext
                .push((b'a' + (10 * encrypt[progress] + encrypt[progress + 1]) as u8 - 1) as char);
            overcome += suffix_num_combs[progress + 1];
            progress += 2;
        }
    }

    println!("{plaintext}");
}

fn valid_encryption(crypt: &[u64]) -> bool {
    match crypt.len() {
        0 => true,
        1 => crypt[0] != 0,
        2 => crypt[0] != 0 && 10 * crypt[0] + crypt[1] <= 26,
        _ => false,
    }
}