import sys
sys.setrecursionlimit(10**6)
N,K=map(int,input().split())
t=list(input())
S=set()
def dfs(s,t):
    global S
    if len(t)==0:
        S.add(tuple(s))
        return
    if len(t)>=2 and t[1]=="0":dfs(s+[int("".join(t[:2]))],t[2:])
    else:dfs(s+[int(t[0])],t[1:])
    if int("".join(t[:2]))<=26:dfs(s+[int("".join(t[:2]))],t[2:])
dfs([],t)
S=sorted(S)
ans=[]
for c in S[K-1]:
    ans.append(chr(ord("a")+c-1))
print("".join(ans))