import sys





def solve():

    readline = sys.stdin.buffer.readline

    mod = 10 ** 9 + 7

    n = int(readline())

    for i in range(1, 50001):

        if int(i * 1.08) == n:

            print(i)

            exit()

    print(":(")





if __name__ == '__main__':

    solve()

import sys





def solve():

    input = sys.stdin.readline

    mod = 10 ** 9 + 7

    n = int(input().rstrip('\n'))

    for i in range(1, 50000):

        if (i * 1.08) // 1 == n:

            print(i)

            exit()

    print(":(")





if __name__ == '__main__':

    solve()

import sys


def solve():
    readline = sys.stdin.buffer.readline
    mod = 10**9 + 7
    n = int(readline())
    for i in range(1, 50001):
        if int(i * 1.08) == n:
            print(i)
            exit()
    print(":(")


if __name__ == "__main__":
    solve()

import sys


def solve():
    input = sys.stdin.readline
    mod = 10**9 + 7
    n = int(input().rstrip("\n"))
    for i in range(1, 50000):
        if (i * 1.08) // 1 == n:
            print(i)
            exit()
    print(":(")


if __name__ == "__main__":
    solve()

-    readline = sys.stdin.buffer.readline +    input = sys.stdin.readline -    n = int(readline()) -    for i in range(1, 50001): -        if int(i * 1.08) == n: +    n = int(input().rstrip("\n")) +    for i in range(1, 50000): +        if (i * 1.08) // 1 == n: