n=int(eval(input()))



ans=-1

for i in range(1,n+1):

  if int(i*1.08)==n:

    ans=i



if ans!=-1:

  print(ans)

else:

  print(":(")
import math

n=int(eval(input()))

for i in range(1,n+1):

  if math.floor(i*1.08)==n:

    print(i)

    break

  else:

    continue

else:

  print(":(")

n = int(eval(input()))
ans = -1
for i in range(1, n + 1):
    if int(i * 1.08) == n:
        ans = i
if ans != -1:
    print(ans)
else:
    print(":(")

import math

n = int(eval(input()))
for i in range(1, n + 1):
    if math.floor(i * 1.08) == n:
        print(i)
        break
    else:
        continue
else:
    print(":(")

+import math + -ans = -1 -    if int(i * 1.08) == n: -        ans = i -if ans != -1: -    print(ans) +    if math.floor(i * 1.08) == n: +        print(i) +        break +    else: +        continue