题目描述

Being a fan of all cold-weather sports (especially those involving cows),Farmer John wants to record as much of the upcoming winter Moolympics as possible.
The television schedule for the Moolympics consists of N different programs(1 <= N <= 150), each with a designated starting time and ending time. FJ has a dual-tuner recorder that can record two programs simultaneously.
Please help him determine the maximum number of programs he can record in total.
农民约翰热衷于所有寒冷天气的运动（尤其是涉及到牛的运动）， 农民约翰想记录尽可能多的moolympics为即将到来的冬季 可能的。 为moolympics电视时间表由N个不同的程序 （1 < = n＝150），每个具有指定的开始时间和结束时间。FJ 有一个双调谐器录音机，可以同时录制两个节目。 请帮助他确定他能录制的节目的最大数量。

输入格式：

Line 1: The integer N.
Lines 2..1+N: Each line contains the start and end time of a single program (integers in the range 0..1,000,000,000).
第1行：整数n。 第2行.. 1 + n：每行包含单个的开始和结束时间 程序（范围为0…1000000000的整数）。

输出格式：

Line 1: The maximum number of programs FJ can record.
仅一行，节目FJ可以记录的最大数量。

输入样例#1：

6
0 3
6 7
3 10
1 5
2 8
1 9

输出样例#1：

4