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

one_list = []
zero_list = []

cur_char = S[0]
char_num = 0
for i in range(N):
    if S[i] == cur_char:
        char_num += 1
    else:
        if cur_char == '0':
            zero_list.append(char_num)
        else:
            one_list.append(char_num)
        cur_char = S[i]
        char_num = 1
if cur_char == '0':
    zero_list.append(char_num)
else:
    one_list.append(char_num)

len_zero_list = len(zero_list)
len_one_list = len(one_list)
# print(len_one_list)
# print(len_zero_list)

cont_length_list = []

if K >= len(zero_list):
    ans = N
else:
    if S[0] == '1':
        tmp_length = sum(one_list[:K+1])+sum(zero_list[:K])
        cont_length_list.append(tmp_length)
        for i in range(len_zero_list-K):
            if K+1+i<len_one_list:
                tmp_length += one_list[K+1+i]+zero_list[K+i]
            else:
                tmp_length += zero_list[K+i]
            tmp_length -= one_list[i]+zero_list[i]
            cont_length_list.append(tmp_length)
        # print(cont_length_list)
    else:
        tmp_length = sum(one_list[:K])+sum(zero_list[:K])
        cont_length_list.append(tmp_length)
        if K < len_zero_list:
            tmp_length += zero_list[K]+one_list[K]
            tmp_length -= zero_list[0]
            cont_length_list.append(tmp_length)
            for i in range(1, len_zero_list-K):
                if K+i<len_one_list and K+i < len_zero_list:
                    tmp_length += one_list[K+i]+zero_list[K+i]
                elif K+i==len_one_list:
                    tmp_length += one_list[K+i]
                else:
                    tmp_length += zero_list[K+i]
                tmp_length -= one_list[i-1]+zero_list[i]
                cont_length_list.append(tmp_length)
        # print(cont_length_list)
    # print(cont_length_list)
    ans = max(cont_length_list)

print(ans)


### FIXED CODE ###

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

one_list = []
zero_list = []

cur_char = S[0]
char_num = 0
for i in range(N):
    if S[i] == cur_char:
        char_num += 1
    else:
        if cur_char == '0':
            zero_list.append(char_num)
        else:
            one_list.append(char_num)
        cur_char = S[i]
        char_num = 1
if cur_char == '0':
    zero_list.append(char_num)
else:
    one_list.append(char_num)

len_zero_list = len(zero_list)
len_one_list = len(one_list)
# print(len_one_list)
# print(len_zero_list)

cont_length_list = []

if K >= len(zero_list):
    ans = N
else:
    if S[0] == '1':
        tmp_length = sum(one_list[:K+1])+sum(zero_list[:K])
        cont_length_list.append(tmp_length)
        for i in range(min(len_zero_list,len_one_list)-K):
            if K+1+i<len_one_list:
                tmp_length += one_list[K+1+i]+zero_list[K+i]
            else:
                tmp_length += zero_list[K+i]
            tmp_length -= one_list[i]+zero_list[i]
            cont_length_list.append(tmp_length)
        # print(cont_length_list)
    else:
        tmp_length = sum(one_list[:K])+sum(zero_list[:K])
        cont_length_list.append(tmp_length)
        if K == len_one_list:
            tmp_length+=zero_list[K]
            tmp_length -= zero_list[0]
            cont_length_list.append(tmp_length)
        if K < len_zero_list-1:
            tmp_length += zero_list[K]+one_list[K]
            tmp_length -= zero_list[0]
            cont_length_list.append(tmp_length)
            for i in range(1, len_zero_list-K):
                if K+i<len_one_list and K+i < len_zero_list:
                    tmp_length += one_list[K+i]+zero_list[K+i]
                else:
                    tmp_length += zero_list[K+i]
                tmp_length -= one_list[i-1]+zero_list[i]
                cont_length_list.append(tmp_length)
        # print(cont_length_list)
    # print(cont_length_list)
    ans = max(cont_length_list)

print(ans)