import math



n = int(eval(input()))



for i in range(1, 50001):

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

        print(i)

        exit()



print(":(")
import math



n = int(eval(input()))



for price in range(1, 50001):

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

        print(price)

        exit()

print(":(")
import math

n = int(eval(input()))
for i in range(1, 50001):
    if i * 1.08 == n or math.floor(i * 1.08) == n:
        print(i)
        exit()
print(":(")

import math

n = int(eval(input()))
for price in range(1, 50001):
    if math.floor(price * 1.08) == n:
        print(price)
        exit()
print(":(")

-for i in range(1, 50001): -    if i * 1.08 == n or math.floor(i * 1.08) == n: -        print(i) +for price in range(1, 50001): +    if math.floor(price * 1.08) == n: +        print(price)