gets
as = gets.split.map(&:to_i).sort
diffs = as.each_cons(2).map {|a, b| b - a}

while true
  m = diffs.min
  i = diffs.index(m)
  x = as.delete_at(i)
  y = as.delete_at(i)
  val = Rational(x + y, 2)
  break if as.size.zero?
  
  j = as.index {|a| a >= val}
  as.insert(j, val)
  diffs = as.each_cons(2).map {|a, b| b - a}
end
puts val.to_f