def main():
    L, K = map(int, input().split())

    haruka = 0
    yu = 0
    while L > 0:
        if L <= 2*K:
            haruka += L
            L = 0
        else:
            haruka += K
            yu += K
            L -= 2*K

    print(yu)

if __name__ == "__main__":  # {{{
    try:
        import test
        test.test()
    except:
        main()  # }}}