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

q=[0]*n+[1]
for i in reversed(range(n)):
  c=0
  if i+1<=n and 1<=int(T[i:i+1])<=26 and T[i]!="0":
    c+=q[i+1]
  if i+2<=n and 1<=int(T[i:i+2])<=26 and T[i]!="0":
    c+=q[i+2]
  q[i]=c

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

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