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

from functools import cache

@cache
def f(x):
  if x==n:
    return 1
  c=0
  if x+1<=n and 1<=int(T[x:x+1])<=26 and T[x]!="0":
    c+=f(x+1)
  if x+2<=n and 1<=int(T[x:x+2])<=26 and T[x]!="0":
    c+=f(x+2)
  return c

S=[]
l=0
while l<n:
  c=f(l+1)
  if K<=c:
    S+=[int(T[l:l+1])]
    l+=1
    continue
  K-=c
  c=f(l+2)
  S+=[int(T[l:l+2])]
  l+=2

print("".join(chr(v-1+ord("a")) for v in S))