N,K=map(int,input().split())
t = input()
inf = 10**20
dp = [0 for _ in range(N+1)]
dp[-1] = 1
for i in range(N-1,-1,-1):
    if t[i] == "0":
        continue
    if i+2<=N and int(t[i]+t[i+1])<=26:
        dp[i] += dp[i+2]
    dp[i] += dp[i+1]
    dp[i] = min(dp[i],inf)
# 最初から決める
i = 0
ans=[]
while i < N:
    if i+2 <= N and t[i+1] == "0":
        ans.append(int(t[i]+t[i+1]))
        i+=2
    if i+2<=N and int(t[i]+t[i+1])<=26:
        if dp[i+1] < K:
            K-=dp[i+1]
            ans.append(int(t[i]+t[i+1]))
            i+=2
        else:
            ans.append(int(t[i]))
            i+=1
    else:
        ans.append(int(t[i]))
        i+=1
print("".join([chr(96+a) for a in ans]))