def get_citations_per_doi(dois_by_year, citations_by_year):
    """
    Returns citations per DOI.
    """
    citations_per_doi = {}
    for year, dois in dois_by_year.items():
        if len(dois) > 0:
            citations_per_doi[year] = round(citations_by_year[year] / len(dois), 2)
    return citations_per_doi