input: Write a function to compute the sum of the values of a given dictionary. If the dictionary is empty, return 0.
  assert dict_sum({'A':5, 'B':6, 'C':10})==21
  assert dict_sum({'X':3, 'Y':7, 'Z':1})==11
  assert dict_sum({})==0
output: def dict_sum(d):
      return sum(d.values()) if d else 0