n,k=map(int,input().split())
t=input()
d=[0]*(1+n)
d[n]=1
for i in range(n)[::-1]:
  if t[i]=='0':
    continue
  d[i]+=d[i+1]
  if 10<int(t[i:i+2])<=26:
    d[i]+=d[i+2]
i=0
ans=[]
while i<n:
  # print(k,i,d[i+1])
  if k<=d[i+1]:
    ans+=chr(int(t[i])+ord('a')-1),
    i+=1
  else:
    ans+=chr(int(t[i:i+2])+ord('a')-1),
    k-=d[i+1]
    i+=2
print(''.join(ans))