input: def uppercase_vowel_count(s):
      """
      Given a string s, determine the number of uppercase vowels located at even indices.
      
      For example:
      uppercase_vowel_count('aBCdEf') returns 1
      uppercase_vowel_count('abcdefg') returns 0
      uppercase_vowel_count('dBBE') returns 0
      """
output:     count = 0
      vowels = {'A', 'E', 'I', 'O', 'U'}
      for i in range(1, len(s), 2):  # check only even index (1-based index is odd, hence 2n+1)
          if s[i] in vowels:
              count += 1
      return count