n, k = map(int, input().split())
t = input()
dp = [0 for _ in range(n+1)]
dp[n] = 1
for i in range(n-1, -1, -1):
    if t[i] == "0":
        continue
    if i < n-1 and int(t[i: i+2]) <= 26:
        dp[i] += dp[i+2]
    dp[i] += dp[i+1]
    dp[i] = min(dp[i], 10**20)

Ans = []
k -= 1
now = 0
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])+96))
        i += 1

print("".join(Ans))