i = gets.to_f
pay = i*100/108
pay = pay.round
if 0 <= pay * 1.08 -i && pay * 1.08 -i < 1
  p pay
elsif 0 <= (pay+1) * 1.08 -i && (pay+1) * 1.08 -i < 1
  p pay+1
elsif 0 <= (pay-1) * 1.08 -i && (pay-1) * 1.08 -i < 1
  p pay-1
else
  print ":("
end