input: Write a python function to calculate the sum of digits in a string.
  assert sum_string("123abc") == 6
  assert sum_string("a1b2c3") == 6
  assert sum_string("abc") == 0
  assert sum_string("4567") == 22
output: def sum_string(s):
      return sum(int(char) for char in s if char.isdigit())