def find_start(dna):
    """
    Finds the location of the start codon.
    dna: a dna sequence
    returns: the location of the first letter of the start codon
    >>> find_start("AAGATGA")
    3
    >>> find_start("AAGATGGATG")
    3
    >>> find_start("ATG")
    0
    """
    for i in range(len(dna)):
        codon = dna[i:i+3] #get a codon
        if codon == 'ATG' and i%3 == 0: #make sure it's a start codon and it's lined up correctly
            return i #tell us the location

        #else:
        #   continue
    return -1