def readinput():

    n=int(eval(input()))

    return n



def main(n):

    x7=int(n/1.07)

    x8=int(n/1.08)

    x9=int(n/1.09)

    n100=n*100

    for x in range(max(1,x9-1),x7+1+1):

        xx=x*108//100

        #print(x,xx)

        if xx==n:

            print(x)

            break

    else:

        print(':(')



        



if __name__=='__main__':

    n=readinput()

    main(n)

def readinput():

    n=int(eval(input()))

    return n



def main(n):

    x7=int(n/1.07)+1

    x9=int(n/1.09)-1

    n100=n*100

    for x in range(max(1,x9),x7+1):

        xx=x*108//100

        #print(x,xx)

        if xx==n:

            print(x)

            break

    else:

        print(':(')



        



if __name__=='__main__':

    n=readinput()

    main(n)

def readinput():
    n = int(eval(input()))
    return n


def main(n):
    x7 = int(n / 1.07)
    x8 = int(n / 1.08)
    x9 = int(n / 1.09)
    n100 = n * 100
    for x in range(max(1, x9 - 1), x7 + 1 + 1):
        xx = x * 108 // 100
        # print(x,xx)
        if xx == n:
            print(x)
            break
    else:
        print(":(")


if __name__ == "__main__":
    n = readinput()
    main(n)

def readinput():
    n = int(eval(input()))
    return n


def main(n):
    x7 = int(n / 1.07) + 1
    x9 = int(n / 1.09) - 1
    n100 = n * 100
    for x in range(max(1, x9), x7 + 1):
        xx = x * 108 // 100
        # print(x,xx)
        if xx == n:
            print(x)
            break
    else:
        print(":(")


if __name__ == "__main__":
    n = readinput()
    main(n)

-    x7 = int(n / 1.07) -    x8 = int(n / 1.08) -    x9 = int(n / 1.09) +    x7 = int(n / 1.07) + 1 +    x9 = int(n / 1.09) - 1 -    for x in range(max(1, x9 - 1), x7 + 1 + 1): +    for x in range(max(1, x9), x7 + 1):