gets
a = gets.split.map{|x| x.to_i << 0}.sort!
l = [nil ,f = [nil, nil]]

a.each { |x|
  l = l[1]
  l[0] = x
  l[1] = [nil,nil]
}
l[1] = nil

while i = f[1]
  x = (f[0] + i[0]) / 2
  f = i[1]
  break if !f && f=[x]
  while n = i[1]
    break  if x >= n[0]
    i = n
  end
  i[1] = [x,n]
  f = i if f[1].object_id == n.object_id
end
p f[0] / (1<<0).to_f