N, K = gets.split.map(&:to_i)
S = gets.chomp

if S.match(/^0+$/) || S.match(/^1+$/)
  puts S.size
  exit
end

f1 = S[0] == '1'

# for 1
b1 = S.split(/0+/).delete_if(&:empty?).map(&:size)

# for 0
b2 = S.split(/1+/).delete_if(&:empty?).map(&:size)

b3sum = [0]

b2.size.times do |i|
  if f1
    s = 0
    s += b3sum[i]
    s += b1[i]
    s += b2[i]
    s += b1[i] if i < b1.size
    b3sum << s
  else
    s = 0
    s += b3sum[i]
    s += b1[i - 1] if i > 0
    s += b2[i]
    s += b1[i] if i < b1.size
    b3sum << s
  end
end

max = 0
max_i = 0

sm = b3sum.size - K - 1
sm = 0 if sm < 0

0.upto(sm) do |i|
  r = i + K
  r = b3sum.size - 1 if b3sum.size <= r
  s = b3sum[r] - b3sum[i]
  if max < s
    max = s
    max_i = i
  end
end

result = 0

sm = max_i + K - 1
sm = b2.size - 1 if b2.size <= sm

max_i.upto(sm) do |i|
  if f1
    result += b1[i] + b2[i]
    if i == sm
      result += b1[i + 1] if b1.size > i + 1
    end
  else
    result += b1[i - 1] if i > 0
    result += b2[i]
    if i == sm
      result += b1[i] if b1.size > i
    end
  end
end

puts result