N = gets.chomp.to_i

X_cand = (N / 1.08).floor
X_array = [X_cand-2, X_cand-1, X_cand, X_cand+1, X_cand+2]

answer = ":("
X_array.each do |x|
  if (x * 1.08).floor == N
    answer = x
    break
  end
end

puts answer