import math



def main():

    N = int(eval(input()))

    ans = False

    for i in range(N+1):

        x = math.floor(i * 1.08)

        if x == N:

            print(i)

            ans = True

            break

    if not ans:

        print(':(')



if __name__ == "__main__":

    main()
import math



def main():

    N = int(eval(input()))

    ans = ':('

    for i in range(N+1):

        x = int(i * 1.08)

        if x == N:

            ans = i

    print(ans)



if __name__ == "__main__":

    main()
import math


def main():
    N = int(eval(input()))
    ans = False
    for i in range(N + 1):
        x = math.floor(i * 1.08)
        if x == N:
            print(i)
            ans = True
            break
    if not ans:
        print(":(")


if __name__ == "__main__":
    main()

import math


def main():
    N = int(eval(input()))
    ans = ":("
    for i in range(N + 1):
        x = int(i * 1.08)
        if x == N:
            ans = i
    print(ans)


if __name__ == "__main__":
    main()

-    ans = False +    ans = ":(" -        x = math.floor(i * 1.08) +        x = int(i * 1.08) -            print(i) -            ans = True -            break -    if not ans: -        print(":(") +            ans = i +    print(ans)