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: Accepted)

package main

import "fmt"

func main() {
	var n, k int

	fmt.Scan(&n)
	fmt.Scan(&k)

	if k <= (n - 1) * (n - 2) / 2 {
		fmt.Println(n * (n - 1) / 2 - k)

		for j := 1; j < n; j++ {			
			fmt.Println(j, n)
		}

		i := k;
		for j1 := 1; j1 < n; j1++ {
			for j2 := j1 + 1; j2 < n; j2++ {
				i++;
				if i <= (n - 1) * (n - 2) / 2 {
					fmt.Println(j1, j2)
				}
			}
		}
	} else {
		fmt.Println(-1)
	}
}