input: def count_upper_vowels(s):
      """
      Given a string s, calculate the number of uppercase vowels present at even positions.
      
      For example:
      count_upper_vowels('aBCdEf') returns 1
      count_upper_vowels('abcdefg') returns 0
      count_upper_vowels('dBBE') returns 0
      """
output:     count = 0
      vowels = 'AEIOU'
      for i in range(1, len(s), 2):
          if s[i] in vowels:
              count += 1
      return count