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

    tmp = L//(2*K)
    tmp1 = L%(2*K)
    if tmp1 == 0:
        print((tmp-1) * K)
    else:
        print(tmp * K)

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