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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

var
	N,K,i,u,v:Longint;
begin
	read(N,K);
	if (N-1)*(N-2)<K*2 then begin
		writeln(-1);
		exit;
	end;
	K:=(N-1)*(N-2)div 2-K;
	writeln(K+N-1);
	for i:=2 to N do begin
		write('1 ');
		writeln(i);
	end;
	u:=2;
	v:=3;
	for i:=1 to K do begin
		write(u);
		write(' ');
		writeln(v);
		inc(v);
		if v>N then begin
			inc(u);
			v:=u+1;
		end;
	end;
end.