Problem:
Score: 500 points

Problem Statement

Does there exist an undirected graph with N vertices satisfying the following conditions?

The graph is simple and connected.

The vertices are numbered 1, 2, ..., N.

Let M be the number of edges in the graph. The edges are numbered 1, 2, ..., M, the length of each edge is 1, and Edge i connects Vertex u_i and Vertex v_i.

There are exactly K pairs of vertices (i,\ j)\ (i < j) such that the shortest distance between them is 2.

If there exists such a graph, construct an example.

Constraints

All values in input are integers.

2 \leq N \leq 100

0 \leq K \leq \frac{N(N - 1)}{2}

Input

Input is given from Standard Input in the following format:

N K

Output

If there does not exist an undirected graph with N vertices satisfying the conditions, print -1.

If there exists such a graph, print an example in the following format (refer to Problem Statement for what the symbols stand for):

M
u_1 v_1
:
u_M v_M

If there exist multiple graphs satisfying the conditions, any of them will be accepted.

Sample Input 1

5 3

Sample Output 1

5
4 3
1 2
3 1
4 5
2 3

This graph has three pairs of vertices such that the shortest distance between them is 2: (1,\ 4), (2,\ 4), and (3,\ 5). Thus, the condition is satisfied.

Sample Input 2

5 8

Sample Output 2

-1

There is no graph satisfying the conditions.

Solution in Go (1.6) (judge verdict: Wrong Answer)

package main

import "fmt"

func main() {
	var n, k int
	fmt.Scan(&n, &k)
	if n < k+2 {
		fmt.Println(-1)
		return
	}
	if n <= 2 {
		fmt.Println(-1)
		return
	}
	mp := map[string]bool{}
	for i := 1; i <= n-1; i++ {
		for j := i + 1; j <= n; j++ {
			mp[fmt.Sprintf("%d %d", i, j)] = true
		}
	}
	for i := 0; i < n-2-k; i++ {
		mp[fmt.Sprintf("%d %d", i+1, i+3)] = false
	}

	fmt.Println(len(mp))
	for k := range mp {
		fmt.Println(k)
	}
}