import sys

input = sys.stdin.readline

sys.setrecursionlimit(10 ** 7)



n = int(eval(input()))



def bs(ok, ng, solve):

    while abs(ok - ng) > 1:

        mid = (ok + ng) // 2

        if solve(mid):

            ok = mid

        else:

            ng = mid

    return ok



ans = bs(ok=n, ng=0, solve=(lambda x: x * 1.08 >= n))

if int(ans * 1.08) == n:

    print(ans)

else:

    print(':(')

import sys

input = sys.stdin.readline

sys.setrecursionlimit(10 ** 7)



n = int(eval(input()))



def bs(ok, ng, solve):

    while abs(ok - ng) > 1:

        mid = (ok + ng) // 2

        if solve(mid):

            ok = mid

        else:

            ng = mid

    return ok



ans = bs(n, 0, lambda x: x * 1.08 >= n)

if int(ans * 1.08) == n:

    print(ans)

else:

    print(':(')

import sys

input = sys.stdin.readline
sys.setrecursionlimit(10**7)
n = int(eval(input()))


def bs(ok, ng, solve):
    while abs(ok - ng) > 1:
        mid = (ok + ng) // 2
        if solve(mid):
            ok = mid
        else:
            ng = mid
    return ok


ans = bs(ok=n, ng=0, solve=(lambda x: x * 1.08 >= n))
if int(ans * 1.08) == n:
    print(ans)
else:
    print(":(")

import sys

input = sys.stdin.readline
sys.setrecursionlimit(10**7)
n = int(eval(input()))


def bs(ok, ng, solve):
    while abs(ok - ng) > 1:
        mid = (ok + ng) // 2
        if solve(mid):
            ok = mid
        else:
            ng = mid
    return ok


ans = bs(n, 0, lambda x: x * 1.08 >= n)
if int(ans * 1.08) == n:
    print(ans)
else:
    print(":(")

-ans = bs(ok=n, ng=0, solve=(lambda x: x * 1.08 >= n)) +ans = bs(n, 0, lambda x: x * 1.08 >= n)