n = gets.chomp.to_i
 
x = (n/1.08).round
n2 = (x * 1.08).floor
n3 = ((x + 1) * 1.08).floor
n4 = ((x - 1) * 1.08).floor
 
if n == n2
    puts x
elsif n == n3
    puts x
elsif n == n4
    puts x
else 
    puts ':('
end