_N = gets.to_i
v = gets.chomp.split.map(&:to_i).sort
i = 0
res = 0
loop do
  break if v.count <= 1
  res = (v[0] + v[1]).to_f / 2.0
  v.slice!(0, 2)
  i += 1
end
if v.empty?
  puts res
else 
  puts (v[0] + res).to_f / 2.0
end