CF827B：High Load（树 & 构造）

最新推荐文章于 2019-04-12 17:51:00 发布

原创最新推荐文章于 2019-04-12 17:51:00 发布 · 470 阅读

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本文探讨了如何构造一个包含特定数量节点的网络，使得度为1的节点间的最大距离最小化。通过采用特定的网络构建策略，可以有效地降低这些关键节点之间的最远距离。

Arkady needs your help again! This time he decided to build his own high-speed Internet exchange point. It should consist of n nodes connected with minimum possible number of wires into one network (a wire directly connects two nodes). Exactly k of the nodes should be exit-nodes, that means that each of them should be connected to exactly one other node of the network, while all other nodes should be connected to at least two nodes in order to increase the system stability.

Arkady wants to make the system as fast as possible, so he wants to minimize the maximum distance between two exit-nodes. The distance between two nodes is the number of wires a package needs to go through between those two nodes.

Help Arkady to find such a way to build the network that the distance between the two most distant exit-nodes is as small as possible.

Input

The first line contains two integers n and k (3 ≤ n ≤ 2·105, 2 ≤ k ≤ n - 1) — the total number of nodes and the number of exit-nodes.

Note that it is always possible to build at least one network with n nodes and k exit-nodes within the given constraints.

Output

In the first line print the minimum possible distance between the two most distant exit-nodes. In each of the next n - 1 lines print two integers: the ids of the nodes connected by a wire. The description of each wire should be printed exactly once. You can print wires and wires' ends in arbitrary order. The nodes should be numbered from 1 to n. Exit-nodes can have any ids.

If there are multiple answers, print any of them.

Examples

input
3 2

output
2
1 2
2 3

input
5 3

output
3
1 2
2 3
3 4
3 5

Note

In the first example the only network is shown on the left picture.

In the second example one of optimal networks is shown on the right picture.

Exit-nodes are highlighted.

$\text{[math]}$

题意：给N个节点，要求构造出一棵树，度为1的点有K个，且这些点的最大距离最小。

思路：以1为圆心，向外扩展K条枝即可。

# include <bits/stdc++.h>
using namespace std;
int main()
{
    int n, k, pre=1;
    scanf("%d%d",&n,&k);
    int c = (int)(ceil((n-k-1)*1.0/k));
    c = c*2+1+((n-k-1)%k>1||(n-k-1)%k==0);
    printf("%d\n",c);
    for(int i=1; i<n; ++i)
    {
        if(i<=k) printf("%d %d\n",pre, i+1);
        else printf("%d %d\n",i+1-k, i+1);
    }
    return 0;
}