gets

h = Hash.new{0}
$stdin.read.split.map(&:to_i).each do |item|
  h[item] += 1
end

count = 0
h.each do |item, num|
  q, r = num.divmod(2)
  count += q
  h[item] = r
end
count += h.values.inject(&:+) / 4

puts count