input: def count_upper(s):
      """
      Given a string s, return the count of uppercase vowels located at even positions.
      
      For example:
      count_upper('aBCdEf') returns 1
      count_upper('abcdefg') returns 0
      count_upper('dBBE') returns 0
      """
output:     count = 0
      upper_vowels = {'A', 'E', 'I', 'O', 'U'}
      for i in range(1, len(s), 2):
          if s[i] in upper_vowels:
              count += 1
      return count