local n = io.read("*n")
n = n * 100
f = false
for i = 1, 50000 do
  local cand = i * 108
  cand = cand - (cand % 100)
  if cand == n then
    print(i)
    f = true
    break
  end
end
if not f then
  print(":(")
end
