def main():

    N = int(eval(input()))

    for x in range(50001):

        if int(x * 1.08) == N:

            print(x)

            exit()



    print(':(')





if __name__ == "__main__":

    main()
def main():

    N = int(eval(input()))

    for i in range(1,50000):

        if int(i * 1.08) == N:

            print(i)

            exit()

        

    print(':(')







if __name__ == "__main__":

    main()
def main():
    N = int(eval(input()))
    for x in range(50001):
        if int(x * 1.08) == N:
            print(x)
            exit()
    print(":(")


if __name__ == "__main__":
    main()

def main():
    N = int(eval(input()))
    for i in range(1, 50000):
        if int(i * 1.08) == N:
            print(i)
            exit()
    print(":(")


if __name__ == "__main__":
    main()

-    for x in range(50001): -        if int(x * 1.08) == N: -            print(x) +    for i in range(1, 50000): +        if int(i * 1.08) == N: +            print(i)