N, K = list(map(int, input().split()))
S = list(input())

L = [1]

for i in range(1, N):
	if S[i] == S[i - 1]:
		L[-1] += 1
	else:
		L.append(1)

n = len(L)
if S[0] == "1":
	if n // 2 <= K:
		print(N)
	else:
		if S[-1] == "1":
			x = L[0]
			for i in range(K):
				x += L[2 * i + 1] + L[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K):
				val += L[2 * (i + K) + 1] + L[2 * (i + K) + 2] - L[2 * i] - L[2 * i + 1]
				if val > x:
					x = val
			print(x)
		else:
			x = L[0]
			for i in range(K):
				x += L[2 * i + 1] + L[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K - 1):
				val += L[2 * (i + K) + 1] + L[2 * (i + K) + 2] - L[2 * i] - L[2 * i + 1]
				if val > x:
					x = val
			val += L[-1] - L[-2 * K] - L[-2 * K - 1]
			if val > x:
				x = val
			print(x)			

else:
	if (n + 1) // 2 <= K:
		print(N)
	else:
		if S[-1] == "1":
			LL = [L[-1 - i] for i in range(n)]
			x = LL[0]
			for i in range(K):
				x += LL[2 * i + 1] + LL[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > x:
					x = val
			print(x)
		else:
			LL = L[1:]
			x = LL[0]
			for i in range(K):
				x += LL[2 * i + 1] + LL[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > x:
					x = val
			val += LL[-1] - LL[-2 * K] - LL[-2 * K - 1]
			if val > x:
				x = val

			LL = [L[-1 - i] for i in range(n - 1)]
			y = LL[0]
			for i in range(K):
				y += LL[2 * i + 1] + LL[2 * i + 2]
			val = y
			for i in range(int(n // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > y:
					y = val
			val += LL[-1] - LL[-2 * K] - LL[-2 * K - 1]
			if val > y:
				y = val
			print(max(x, y))	

### FIXED CODE ###

N, K = list(map(int, input().split()))
S = list(input())

L = [1]

for i in range(1, N):
	if S[i] == S[i - 1]:
		L[-1] += 1
	else:
		L.append(1)

n = len(L)
if S[0] == "1":
	if n // 2 <= K:
		print(N)
	else:
		if S[-1] == "1":
			x = L[0]
			for i in range(K):
				x += L[2 * i + 1] + L[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K):
				val += L[2 * (i + K) + 1] + L[2 * (i + K) + 2] - L[2 * i] - L[2 * i + 1]
				if val > x:
					x = val
			print(x)
		else:
			x = L[0]
			for i in range(K):
				x += L[2 * i + 1] + L[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K - 1):
				val += L[2 * (i + K) + 1] + L[2 * (i + K) + 2] - L[2 * i] - L[2 * i + 1]
				if val > x:
					x = val
			val += L[-1] - L[-2 * K - 1] - L[-2 * K - 2]
			if val > x:
				x = val
			print(x)			

else:
	if (n + 1) // 2 <= K:
		print(N)
	elif (n - 1) // 2 == K and S[0] == "0" and S[-1] == "0":
		print(max(N - L[0], N - L[-1]))
	else:
		if S[-1] == "1":
			LL = [L[-1 - i] for i in range(n)]
			x = LL[0]
			for i in range(K):
				x += LL[2 * i + 1] + LL[2 * i + 2]
			val = x
			for i in range(int(n // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > x:
					x = val
			val += L[0] - LL[-2 * K + 1] - LL[-2 * K]
			if val > x:
				x = val
			print(x)
		else:
			LL = L[1:]
			x = LL[0]
			for i in range(K):
				x += LL[2 * i + 1] + LL[2 * i + 2]
			val = x
			for i in range(int((n - 1) // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > x:
					x = val
			val += L[-1] - LL[-2 * K + 1] - LL[-2 * K]
			if val > x:
				x = val

			LL = [L[-2 - i] for i in range(n - 1)]
			y = LL[0]
			for i in range(K):
				y += LL[2 * i + 1] + LL[2 * i + 2]
			val = y
			for i in range(int((n - 1) // 2) - K - 1):
				val += LL[2 * (i + K) + 1] + LL[2 * (i + K) + 2] - LL[2 * i] - LL[2 * i + 1]
				if val > y:
					y = val
			val += L[0] - LL[-2 * K + 1] - LL[-2 * K]
			if val > y:
				y = val
			print(max(x, y))