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

def code(s):
    return ord(s)-ord("a")
def codeR(n):
    return chr(ord("a")+n-1)

dp = [0]*(N+1)
dp[-1] = 1
limit = K
for i in reversed(range(N)):
    if T[i] == "0":
        continue
    dp[i] = dp[i+1]
    if i+1 < N and int(T[i:i+2]) <= 26:
        dp[i] += dp[i+2]
        dp[i] = min(dp[i], limit)

ans = []
SUM = 0
idx = 0
while idx < N:
    if K <= SUM+dp[idx+1]:
        ans.append(codeR(int(T[idx])))
        idx += 1
    else:
        ans.append(codeR(int(T[idx:idx+2])))
        SUM += dp[idx+1]
        idx += 2

print("".join(ans))