区间相交问题给定x 轴上n 个闭区间。去掉尽可能少的闭区间，使剩下的闭区间都不相交。

由文件input.txt给出输入数据。第一行是正整数n，表示闭区间数。接下来的n行中，

每行有2 个整数，分别表示闭区间的2个数端点。-Interval intersection problem of a given x-axis n-closed interval. Remove the closed interval of as little as possible, so that the rest do not intersect the closed interval. Given by the input data file input.txt. The first line is a positive integer n, that the number of closed interval. N the next line, each line has two integers, respectively, the number of closed-end interval 2.