N,K=map(int,input().split())
S=input()
p=[*map(int,S+S)]
q=2*N*[0]
m=0
for i in range(2*N):
	if m:
		m-=1
		q[i]=1
	m+=p[i]
p=q[N:]
n=[(K//N-1)*sum(q[N:])+sum(q[:K%N+N]),sum(q[:K])][K<N]
print(K-n)