n,A=map(int,input().split())
s=input()
a=0
c=0
for i in range(n):
    if s[i]=='0':
        a+=1 
    else:
        if a>0:
            a-=1 
            c+=1 
    if c==A:
        print(i+1)
        exit()
x=s.count('0')
y=s.count('1')
if x>=y:
    z=max(1,(A+y-c-1)//y-3)
    d=(x-y)*(z-1)+x-c
    A-=c*z+(y-c)*(z-1)
    ans=z*n
else:
    z=max(1,(A+x-c)//x-3)
    d=x-c 
    A-=c*z+(x-c)*(z-1)
    ans=z*n
i=0
while True:
    if A==0:
        break 
    ans+=1 
    if s[i%n]=='0':
        d+=1 
    else:
        if d>0:
            d-=1 
            A-=1
    i+=1
print(ans)