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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define ll long long
using namespace std;

int dist[110][110];

int main() {
    int n, k;
    cin >> n >> k;
    if (k > (n - 2) * (n - 1) / 2) {
        cout << -1 << endl;
        return 0;
    }
    int ma = (n - 2) * (n - 1) / 2;
    int sub = ma - k;
    vector<int> v[n + 1];
    int m = 0;
    for (int i = 2; i <= n; i++) {
        v[1].push_back(i);
        m++;
    }
    int idx = n - 1;
    while (sub > 0) {
        for (int j = idx; j < n; j++) {
            v[idx].push_back(j + 1);
            m++;
            sub--;
            if (sub == 0) break;
        }
        idx--;
    }
    cout << m << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 0; j < v[i].size(); j++) {
            cout << i << " " << v[i][j] << endl;
        }
    }
    return 0;
}