n = gets.to_i
s = gets.chomp.chars.map(&:to_i)
a = 0
z = Array.new(n)

z.fill(0)
s.each_cons(2).with_index do |(x, y), i|
  z[i] += 1 if y == 0
  z[i + 1] += 1 if x == 0
end

(0...(n - 2)).each do |i|
  next unless s[i] == 0 && s[i + 1] == 1 && s[i + 2] == 0

  a += 1
  nxi = z[i] > z[i + 2] ? i : i + 2
  s[nxi] = 1
  z[nxi - 1] -= 1 if nxi > 1
  z[nxi + 1] -= 1 if nxi < n - 1
end

(0...(n - 1)).each do |i|
  next unless s[i] == 0 && s[i + 1] == 0

  a += 1
  nxi = z[i] > z[i + 1] ? i : i + 1
  s[nxi] = 1
  z[nxi - 1] -= 1 if nxi > 1
  z[nxi + 1] -= 1 if nxi < n - 1
end

(0...(n - 2)).each do |i|
  next unless s[i] == 0 && s[i + 1] == 1 && s[i + 2] == 0

  a += 1
  nxi = z[i] > z[i + 2] ? i : i + 2
  s[nxi] = 1
  z[nxi - 1] -= 1 if nxi > 1
  z[nxi + 1] -= 1 if nxi < n - 1
end

puts a