n, k = map(int, input().split())
k -= 1

s = [int(c) for c in input()]

dp = [0] * (n + 1)
dp[n] = 1
for i in range(n)[::-1]:
    if s[i] == 0:
        continue
    dp[i] = dp[i+1]
    if i + 2 <= n and 10 * s[i] + s[i+1] <= 26:
        dp[i] = min(dp[i] + dp[i+2], k + 1)

ans = []
i = 0
while i < n:
    assert s[i] != 0
    if dp[i+1] <= k:
        assert i + 2 <= n
        k -= dp[i+1]
        ans.append(chr(96 + 10 * s[i] + s[i+1]))
        i += 2
    else:
        ans.append(chr(96 + s[i]))
        i += 1
print(''.join(ans))