input: Write a function to compute the sum of the values of a given dictionary. If the dictionary is empty, return 0.
  dict_sum({'A': 5, 'B': 6, 'C': 10})==21
  dict_sum({'X': 3, 'Y': 7, 'Z': 12})==22
  dict_sum({})==0
output: def dict_sum(d):
      return sum(d.values())