题目描述

Ever the maturing businessman, Farmer John realizes that he must manage his time effectively. He has N jobs conveniently numbered 1..N (1 <= N <= 1,000) to accomplish (like milking the cows, cleaning the barn, mending the fences, and so on).
To manage his time effectively, he has created a list of the jobs that must be finished. Job i requires a certain amount of time T_i (1 <= T_i <= 1,000) to complete and furthermore must be finished by time S_i (1 <= S_i <= 1,000,000). Farmer John starts his day at time t=0 and can only work on one job at a time until it is finished.
Even a maturing businessman likes to sleep late; help Farmer John determine the latest he can start working and still finish all the jobs on time.
作为一名忙碌的商人，约翰知道必须高效地安排他的时间.他有N工作要 做，比如给奶牛挤奶，清洗牛棚，修理栅栏之类的.
为了高效，列出了所有工作的清单.第i分工作需要T_i单位的时间来完成，而 且必须在S_i或之前完成.现在是0时刻.约翰做一份工作必须直到做完才能停 止.
所有的商人都喜欢睡懒觉.请帮约翰计算他最迟什么时候开始工作，可以让所有工作按时完 成.（如果无法完成全部任务，输出-1）

输入格式：

Line 1: A single integer: N
Lines 2..N+1: Line i+1 contains two space-separated integers: T_i and S_i
第 1 行：单个整数：N
第 2..N+1 行：第 i+1 行包含两个空格分隔的整数：T_i 和 S_i

输出格式：

Line 1: The latest time Farmer John can start working or -1 if Farmer John cannot finish all the jobs on time.
第 1 行：Farmer John 可以开始工作的最晚时间，如果 Farmer John 不能按时完成所有工作，则为 -1。

输入样例#1：

4 
3 5 
8 14 
5 20 
1 16 

输出样例#1：

2