study_exam1.rb

cnt =0
inputs = []
while str = STDIN.gets
  inputs << str.to_i
end
inputs.each do |i|
  
 inputs2 = inputs.dup
 inputs2.delete(i)
 
  inputs2.each do | j |
    
    if (i.to_i + j.to_i) == 0 then
     inputs.delete(i)
      cnt = cnt + 1
    end
  end
end
puts cnt