def accentuate(letter):
    """
    Function that adds the accentuation mark of a letter
    """
    if letter == "a":
        letter = "á"
    elif letter == "e":
        letter = "é"
    elif letter == "i":
        letter = "í"
    elif letter == "o":
        letter = "ó"
    elif letter == "u":
        letter = "ú"
    return letter