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