from math import floor



def main(N, rate=1.08):

    ans = -1

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

        if floor(x * 1.08) == N:

            ans = x

    return ans





if __name__ == "__main__":

    N = int(eval(input()))

    ans = main(N)

    print((ans if ans > 0 else ':('))

def main(N, rate=1.08):

    X = int(-(-N // 1.08))

    return X if int(X * rate) == N else -1





if __name__ == "__main__":

    N = int(eval(input()))

    ans = main(N)

    print((ans if ans > 0 else ':('))

from math import floor


def main(N, rate=1.08):
    ans = -1
    for x in range(1, N + 1):
        if floor(x * 1.08) == N:
            ans = x
    return ans


if __name__ == "__main__":
    N = int(eval(input()))
    ans = main(N)
    print((ans if ans > 0 else ":("))

def main(N, rate=1.08):
    X = int(-(-N // 1.08))
    return X if int(X * rate) == N else -1


if __name__ == "__main__":
    N = int(eval(input()))
    ans = main(N)
    print((ans if ans > 0 else ":("))

-from math import floor - - -    ans = -1 -    for x in range(1, N + 1): -        if floor(x * 1.08) == N: -            ans = x -    return ans +    X = int(-(-N // 1.08)) +    return X if int(X * rate) == N else -1