N,K=map(int,input().split())
S=input()
h=[0]*(N+1)
x=0
E=[0]*(N+1)
for i in range(N):
  if S[i]=='1':
    x+=1
    E[i+1]=E[i]+1
  else:
    x-=1
    E[i+1]=E[i]
  h[i+1]=max(h[i],x)
P=x
t=max(h)
l=1
r=2*10**18
z=E[N]
while True:
  if l==r:
    break
  m=(l+r)//2
  count=m//N
  pos=m%N
  d=max(count*x+h[pos],h[pos])
  if count>0:
    d=max(d,(count-1)*x+t,t)
  score=z*count+E[pos]-d
  if score>=K:
    r=m
  else:
    l=m+1
print(l)