某操作系统的内存管理采用段式管理，内存分配采用最佳适配算法，计算机的内存为50M，刚开始时内存均未使用。经过一段时间的运行(其内存申请；释放的序列为：申请20M一申请15M一申请10M一释放15M一申请10M一释放20M一申请4M)以后，内存中存在()。

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