ch0406-2390 Pearl Pairing

描述

At Bessie’s recent birthday party, she received N (2 <= N <= 100,000; N%2 == 0) pearls, each painted one of C different colors (1 <= C <= N).Upon observing that the number of pearls N is always even, her creative juices flowed and she decided to pair the pearls so that each pair of pearls has two different colors.Knowing that such a set of pairings is always possible for the supplied testcases, help Bessie perform such a pairing. If there are multiple ways of creating a pairing, any solution suffices.

输入

• Line 1: Two space-separated integers: N and C* Lines 2…C + 1: Line i+1 tells the count of pearls with color i: C_i

输出

• Lines 1…N/2: Line i contains two integers a_i and b_i indicating that Bessie can pair two pearls with respective colors a_i and b_i.

样例输入

1
2
3
48 3
2
2
4

样例输出

1
2
3
41 3
1 3
2 3
3 2

提示

INPUT DETAILS:There are 8 pearls and 3 different colors. Two pearls have color I; two have color II; four have color III.OUTPUT DETAILS:Bessie pairs each pearl of color III with one of color I and II.

来源

USACO March 2008 Gold

限制

• 时间限制：1000ms
• 内存限制：128MB