line = readlines
len = line.length
i = 0

while i < len
    line[i] = line[i].chomp.split(' ')
    i += 1
end

x = line[0][0]
y = x.to_f/1.08.to_f
if ((y.to_i)*1.08).to_i == x then
  puts y.to_i
else
  puts ":("
end