fn getline() -> String {
    let mut ret = String::new();
    std::io::stdin().read_line(&mut ret).unwrap();
    ret
}

fn main() {
    let ints = getline()
        .split_whitespace()
        .map(|x| x.parse::<i64>().unwrap())
        .collect::<Vec<_>>();
    let k = ints[1];
    let t = getline().trim().bytes().map(|i| i - b'0').collect::<Vec<_>>();
    let n = t.len();
    let mut dp = vec![0i64; n + 1];
    dp[n] = 1;
    for i in (0..n).rev() {
        let mut me = 0i64;
        if t[i] != 0 {
            me = me.saturating_add(dp[i + 1]);
        }
        if i + 2 <= n {
            let val = t[i] * 10 + t[i + 1];
            if 10 <= val && val <= 26 {
                me = me.saturating_add(dp[i + 2]);
            }
        }
        dp[i] = me.min(k + 1);
    }
    let mut ans = "".to_string();
    let mut rem = k - 1;
    let mut i = 0;
    while i < n {
        let one = if t[i] == b'0' { 0 } else { dp[i + 1] };
        if rem < one {
            ans.push((b'a' + t[i] - 1) as char);
        } else {
            rem -= one;
            let val = if i + 2 <= n {
                t[i] * 10 + t[i + 1]
            } else {
                panic!()
            };
            if 10 <= val && val <= 26 {
                ans.push((b'a' + val - 1) as char);
                i += 1;
            } else {
                panic!();
            }
        }
        i += 1;
    }
    assert_eq!(rem, 0);
    println!("{ans}");
}