def lettergrade(value):
    """
    Maps grade point average to letter grade.
    """
    if value > 3.85:
        return 'A'
    elif value > 3.5:
        return 'A-'
    elif value > 3.15:
        return 'B+'
    elif value > 2.85:
        return 'B'
    elif value > 2.5:
        return 'B-'
    elif value > 2.15:
        return 'C+'
    elif value > 1.85:
        return 'C'
    elif value > 1.5:
        return 'C-'
    elif value > 1.15:
        return 'D+'
    elif value > 0.85:
        return 'D'
    elif value > 0.5:
        return 'D-'
    else:
        return 'F'