N=int(eval(input()))

l=N*100//108

stp=0

for i in range(2):

  if (l+i)*108//100==N:

    print((l+i))

    stp=1

    break

if stp==0:

  print(':(')
# coding: utf-8



N = int(eval(input()))



x_ = N//1.08

ans = ':('

for i in range(1,100000):

    x = x_+i

    if int(1.08*x) == N:

        ans = int(x)

        break



print(ans)
N = int(eval(input()))
l = N * 100 // 108
stp = 0
for i in range(2):
    if (l + i) * 108 // 100 == N:
        print((l + i))
        stp = 1
        break
if stp == 0:
    print(":(")

# coding: utf-8
N = int(eval(input()))
x_ = N // 1.08
ans = ":("
for i in range(1, 100000):
    x = x_ + i
    if int(1.08 * x) == N:
        ans = int(x)
        break
print(ans)

+# coding: utf-8 -l = N * 100 // 108 -stp = 0 -for i in range(2): -    if (l + i) * 108 // 100 == N: -        print((l + i)) -        stp = 1 +x_ = N // 1.08 +ans = ":(" +for i in range(1, 100000): +    x = x_ + i +    if int(1.08 * x) == N: +        ans = int(x) -if stp == 0: -    print(":(") +print(ans)