# Input the amount of money Takahashi paid

N = int(eval(input()))



# Check if the price before tax is X, for X = 1, 2, 3, ..., N

ans = -1

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

	if int(i * 1.08) == N:

		ans = i



# Print answer

if ans != -1:

	print(ans)

else:

	print(":(")
import math



# Input the amount of money Takahashi paid

N = int(eval(input()))



# Find nearest integer price more than or equal to X/1.08

X = math.ceil(N / 1.08)



# Check if the "taxed price" is equal to N yen

if math.floor(X * 1.08) == N:

	print(X)

else:

	print(":(")
# Input the amount of money Takahashi paid
N = int(eval(input()))
# Check if the price before tax is X, for X = 1, 2, 3, ..., N
ans = -1
for i in range(1, N + 1):
    if int(i * 1.08) == N:
        ans = i
# Print answer
if ans != -1:
    print(ans)
else:
    print(":(")

import math

# Input the amount of money Takahashi paid
N = int(eval(input()))
# Find nearest integer price more than or equal to X/1.08
X = math.ceil(N / 1.08)
# Check if the "taxed price" is equal to N yen
if math.floor(X * 1.08) == N:
    print(X)
else:
    print(":(")

+import math + -# Check if the price before tax is X, for X = 1, 2, 3, ..., N -ans = -1 -for i in range(1, N + 1): -    if int(i * 1.08) == N: -        ans = i -# Print answer -if ans != -1: -    print(ans) +# Find nearest integer price more than or equal to X/1.08 +X = math.ceil(N / 1.08) +# Check if the "taxed price" is equal to N yen +if math.floor(X * 1.08) == N: +    print(X)