Codeforces 417 C (递推)

Football

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Description

One day, at the "Russian Code Cup" event it was decided to play football as an out of competition event. All participants was divided into nteams 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 ktimes. 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 and k (1 ≤ n, k ≤ 1000).

Output

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

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

Sample Input

Input

3 1

Output

31 22 33 1

Source

RCC 2014 Warmup (Div. 2)

很久之前做的题，输出所有的情况就行了。

#include <iostream>
#include <stdio.h>
typedef long long LL;
using namespace std;
int main()
{
    int i,n,m,k,j;
    cin>>n>>k;
    if (n-1<2*k)
    cout<<-1<<endl;
    else
    {
        cout<<n*k<<endl;
        for (i=1;i<=n;i++)
        {
            for (j=0;j<k;j++)
            printf("%d %d\n",i,(i+j)%n+1);
        }
    }
    return 0;
}