说明：  某校大门外长度为L的马路上有一排树，每两棵相邻的树之间的间隔都是1米。我们可以把马路堪称一个数轴，马路的一端在数轴0的位置，另一端在L的位置；数轴上的每个整数点，即0,1,2,……,L,都种有一棵树。由于马路上的一些区域中的始点和终点的坐标都是整数，区域之间可能有重合部分。现在要把这些区域中的树（包括2端点）移走。计算还剩多少树？
(There are several test cases in the input. Each case starts with an integer L (1 <= L < 2000000000) representing the length of the road and M (1 <= M <= 5000) representing the number of sections that are assigned for other use.  The following M lines each describes a section. A line is in such format:  Start End  Here Start and End (0 <= Start <= End <= L) are both non-negative integers representing the start point and the end point of the section. It is confirmed that these sections do not overlap with each other.  A case with L = 0 and M = 0 ends the input.)