cf 417 C Football

题目链接：http://codeforces.com/problemset/problem/417/C

C. Football

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

One day, at the "Russian Code Cup" event it was decided to play football as an out of competition event. All participants was divided inton teams and played several matches, two teams could not play against each other more than once.

The appointed Judge was the most experienced member — Pavel. But since he was the wisest of all, he soon got bored of the game and fell asleep. Waking up, he discovered that the tournament is over and the teams want to know the results of all the matches.

Pavel didn't want anyone to discover about him sleeping and not keeping an eye on the results, so he decided to recover the results of all games. To do this, he asked all the teams and learned that the real winner was friendship, that is, each team beat the other teams exactly k times. Help Pavel come up with chronology of the tournir that meets all the conditions, or otherwise report that there is no such table.

Input

The first line contains two integers — n andk (1 ≤ n, k ≤ 1000).

Output

In the first line print an integer m — number of the played games. The followingm lines should contain the information about all the matches, one match per line. Thei-th line should contain two integers a_i and b_i (1 ≤ a_i, b_i ≤ n;a_i ≠ b_i). The numbersa_i andb_i mean, that in thei-th match the team with number a_i won against the team with numberb_i. You can assume, that the teams are numbered from1 to n.

If a tournir that meets the conditions of the problem does not exist, then print -1.

Examples

Input

3 1

Output

31 22 33 1

题意：

1.n个足球队伍，每个队伍赢得k场比赛

2.输出比赛次数以及每次的胜负方。
3.如果没有满足的情况就输出-1

不过并不是一定每两个队都有比赛，两个队之间可能比赛也可能不比赛，当然最多也就只有n*(n-1)/2场比赛咯

思路：

创建一个长度为2n的数组，将1-n储存两边，以k为一个循环标准，依次遍历输出

代码：

#define _CRT_SBCURE_MO_DEPRECATE    #include<iostream>    #include<stdlib.h>    #include<stdio.h>    #include<cmath>    #include<algorithm>    #include<string>    #include<string.h>    #include<set>    #include<queue>    #include<stack>    #include<functional>     using namespace std;const int maxn = 10000 + 10;const int INF = 0x3f3f3f3f;int n, k;int s[maxn];int t, m;int main(){while (scanf("%d %d", &n, &k) != EOF) {if ((n*(n - 1) / 2 < n*k)) {printf("-1\n");continue;}printf("%d\n", n*k);for (int i = 0; i <= 2 * n; i++) {if (i <= n)s[i] = i;else s[i] = s[i - n];}//m = k;t = 1;m = 1 + k;t = 2;for (int i = 1; i <= n; i++){for (int j = t; j <= m; j++)printf("%d %d\n", i, s[j]);m++;t++;}}//system("pause");return 0;}

silu