<span><span>某校大门外长度为</span><span><span><span>L</span></span></span></span><span><span>的马路上有一排树，每两棵相邻的树之间的间隔都是</span><span><span><span>1</span></span></span></span><span><span>米。我们可以把马路看成一个数轴，马路的一端在数轴</span><span><span><span>0</span></span></span></span><span><span>的位置，另一端在</span><span><span><span>L</span></span></span></span><span><span>的位置；数轴上的每个整数点，即</span><span><span><span>0</span></span></span></span><span><span>，</span><span><span><span>1</span></span></span></span><span><span>，</span><span><span><span>2</span></span></span></span><span><span>，……，</span><span><span><span>L</span></span></span></span><span><span>，都种有一棵树。</span></span><span><span><span><span><br />

由于马路上有一些区域要用来建地铁。这些区域用它们在数轴上的起始点和终止点表示。已知任一区域的起始点和终止点的坐标都是整数，区域之间可能有重合的部分。现在要把这些区域中的树（包括区域端点处的两棵树）移走。你的任务是计算将这些树都移走后，马路上还有多少棵树。

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<span><span>输入文件</span><span><span><span>tree.in</span></span></span></span><span><span>的第一行有两个整数</span><span><span><span>L</span></span></span></span><span><span>（</span><span><span><span>1 &lt;= L &lt;= 10000</span></span></span></span><span><span>）和 </span><span><span><span>M</span></span></span></span><span><span>（</span><span><span><span>1 &lt;= M &lt;= 100</span></span></span></span><span><span>），</span><span><span><span>L</span></span></span></span><span><span>代表马路的长度，</span><span><span><span>M</span></span></span></span><span><span>代表区域的数目，</span><span><span><span>L</span></span></span></span><span><span>和</span><span><span><span>M</span></span></span></span><span><span>之间用一个空格隔开。接下来的</span><span><span><span>M</span></span></span></span><span><span>行每行包含两个不同的整数，用一个空格隔开，表示一个区域的起始点和终止点的坐标。</span></span>

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<span><span>输出文件</span><span><span><span>tree.out</span></span></span></span><span><span>包括一行，这一行只包含一个整数，表示马路上剩余的树的数目。</span></span>

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