N = gets.chomp.to_i
# A, B, C = gets.chomp.split.map(&:to_i)
# S = gets.chomp.chars

def solve
  x = (1.0*N) / 1.08
  x1 = x.ceil
  x2 = x.floor

  if (x1 * 1.08).floor == N
    p x1
    return
  end
  if (x2 * 1.08).floor == N
    p x2
    return
  end
  
  print ":(\n"
end

solve
