[USACO07JAN] Problem Solving G

题目描述

In easier times, Farmer John's cows had no problems. These days, though, they have problems, lots of problems; they have P (1 ≤ P ≤ 300) problems, to be exact. They have quit providing milk and have taken regular jobs like all other good citizens. In fact, on a normal month they make M (1 ≤ M ≤ 1000) money.

Their problems, however, are so complex they must hire consultants to solve them. Consultants are not free, but they are competent: consultants can solve any problem in a single month. Each consultant demands two payments: one in advance (1 ≤ payment ≤ M) to be paid at the start of the month problem-solving is commenced and one more payment at the start of the month after the problem is solved (1 ≤ payment ≤ M). Thus, each month the cows can spend the money earned during the previous month to pay for consultants. Cows are spendthrifts: they can never save any money from month-to-month; money not used is wasted on cow candy.

Since the problems to be solved depend on each other, they must be solved mostly in order. For example, problem 3 must be solved before problem 4 or during the same month as problem 4.

Determine the number of months it takes to solve all of the cows' problems and pay for the solutions.

P个问题，雇佣相同的人去解决，每个人每月解决一道题，每个人解决问题的代价都分两次，解决问题当月给a[i],事后第二月给b[i],然后每个月有m的钱，问最快多久解决所有问题。（问题必须按照序号一个个解决） 这个月用上个月的钱支付，然后结余会用来买糖

输入格式

Line 1: Two space-separated integers: M and P.

Lines 2..P+1: Line i+1 describes problem i with two space-separated integers: Bi and Ai. Bi is the payment to the consult BEFORE the problem is solved; Ai is the payment to the consult AFTER the problem is solved.

输出格式

Line 1: The number of months it takes to solve and pay for all the cows' problems.

样例 #1

样例输入 #1

100 5
40 20
60 20
30 50
30 50
40 40

样例输出 #1

6

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