[USACO06JAN] Corral the Cows G

Farmer John wishes to build a corral for his cows. Being finicky beasts, they demand that the corral be square and that the corral contain at least C (1 <= C <= 500) clover fields for afternoon treats. The corral's edges must be parallel to the X,Y axes. FJ's land contains a total of N (C <= N <= 500) clover fields, each a block of size 1 x 1 and located at with its lower left corner at integer X and Y coordinates each in the range 1..10,000. Sometimes more than one clover field grows at the same location; such a field would have its location appear twice (or more) in the input. A corral surrounds a clover field if the field is entirely located inside the corral's borders. Help FJ by telling him the side length of the smallest square containing C clover fields. Farmer John 希望为他的奶牛们建立一个畜栏。 这些挑剔的奶牛要求畜栏必须是正方形的，而且至少要包含 $C$ 片三叶草，来当做它们的下午茶。 约翰的土地里一共包含 $N$ 片三叶草，每单位三叶草位于一个 $1\times 1$ 的土地区域内。坐标都为整数，范围在 $1$ 到 $10000$ 以内。 **多片的三叶草可能会位于同一个 $1\times 1$ 的区域内，所以在输入内，同一个区域坐标可能出现多次。** 请你帮 Farmer John 计算一下，在能包含至少 $C$ 片三叶草的情况下，畜栏的最小边长是多少。 $1\le C\le N\le 500$。

Line 1: Two space-separated integers: C and N Lines 2..N+1: Each line contains two space-separated integers that are the X,Y coordinates of a clover field.

Line 1: A single line with a single integer that is length of one edge of the minimum size square that contains at least C clover fields.

3 4
1 2
2 1
4 1
5 2

4

Explanation of the sample: |\* \* | \* \* +------Below is one 4x4 solution (C's show most of the corral's area); many others exist. |CCCC |CCCC |\*CCC\* |C\*C\* +------