Simple sum of pairs

def digit_sum(n):
   r = 0
   while n:
       r, n = r + n % 10, n // 10
   return r


def solve(n):
    out = 0
    f = 0
    for i in range(round(n/2)+1):
        f = digit_sum(i)+digit_sum(n - i)
        if f > out:
            out = f
    print(str(out))

    return out