def count_vowels(s):
    """Returns the number of letters 'a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U'
    in a given string s"""

    letters = "aeiouAEIOU"
    count = 0
    for letter in s:
        if letter in letters:
            count += 1
    return count