n = int(eval(input()))

xx = (100 * n // 108) + 1

# print(xx)

for x in range(xx + 1):

    nn = int(x * 1.08 // 1)

    if n == nn:

        print(x)

        exit(0)

print(':(')

import math



n = int(eval(input()))

x = math.ceil(n / 1.08)



if n == int(x * 1.08):

  print(x)

else:

  print(':(')

n = int(eval(input()))
xx = (100 * n // 108) + 1
# print(xx)
for x in range(xx + 1):
    nn = int(x * 1.08 // 1)
    if n == nn:
        print(x)
        exit(0)
print(":(")

import math

n = int(eval(input()))
x = math.ceil(n / 1.08)
if n == int(x * 1.08):
    print(x)
else:
    print(":(")

+import math + -xx = (100 * n // 108) + 1 -# print(xx) -for x in range(xx + 1): -    nn = int(x * 1.08 // 1) -    if n == nn: -        print(x) -        exit(0) -print(":(") +x = math.ceil(n / 1.08) +if n == int(x * 1.08): +    print(x) +else: +    print(":(")