d=gets.to_i
s=Array.new(2){gets.chomp}.inject{|s, t| s+t}

d.times{s='x'+s+'x'}
s=s.chars.each.map{|c| c=='x' ? 0 : 1}
mx=0
(s.size-d+1).times do |i|
  next unless s[i...i+d].inject(:+).zero?
  for j in i...i+d do
    s[j]^=1
  end
  l, r, cnt=0, 0, 0
  while r<s.size
    l=r
    while r<s.size and s[r].zero?
      l+=1
      r+=1
    end
    while r<s.size and s[r]==1
      r+=1
    end
    cnt=[cnt, r-l].max
  end
  # print "#{i} #{cnt}\n"
  mx=[mx, cnt].max
  for j in i...i+d do
    s[j]^=1
  end
end

puts mx