from string import ascii_lowercase
import sys
import os


IS_LOCAL = os.environ.get("LOCAL") == "true"


def debug(*args, sep=" ", end="\n", flush=False) -> None:
    if IS_LOCAL:
        print(*args, sep=sep, end=end, file=sys.stderr, flush=flush)


def yn(flg: bool) -> bool:
    print('Yes' if flg else 'No')
    return flg


def main():
    readline = sys.stdin.readline
    inf = 10**18 + 9

    N, K = map(int, readline().split())
    K -= 1
    S = readline().strip()
    dp = [0] * (N + 1)
    dp[N] = 1
    for i in range(N - 1, -1, -1):
        c = int(S[i])
        if c == 0:
            continue
        dp[i] = dp[i + 1]
        if c <= 2 and i + 1 < N and 1 <= int(S[i:i + 2]) <= 26:
            dp[i] += dp[i + 2]
        dp[i] = min(dp[i], inf)
    debug(dp)
    ans = []
    i = 0
    while i < N:
        c = int(S[i])
        if c <= 2 and i + 1 < N and 1 <= int(S[i:i + 2]) <= 26 and K >= dp[i + 1]:
            K -= dp[i + 1]
            c = int(S[i:i + 2])
            i += 2
        else:
            i += 1
        ans.append(ascii_lowercase[c - 1])
    print("".join(ans))


if __name__ == "__main__":
    main()