N, K = map(int, input().split())
t = input()

dp = [0] * (N + 1)
dp[N] = 1

for i in range(N - 1, -1, -1):
    if 1 <= int(t[i : i + 1]) <= 9:
        dp[i] += dp[i + 1]
    if i + 2 <= N and 10 <= int(t[i : i + 2]) <= 26:
        dp[i] += dp[i + 2]

ans = []
i = 0
K -= 1
while i < N:
    if dp[i + 1] <= K:
        K -= dp[i + 1]
        ans.append(chr(int(t[i : i + 2]) + 96))
        i += 2
    else:
        ans.append(chr(int(t[i : i + 1]) + 96))
        i += 1

print("".join(ans))