#include "bits/stdc++.h"

using namespace std;
#ifdef _DEBUG
#define _GLIBCXX_DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define ll long long
#define DBG 1
#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define rrep(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define loop(n) rep(loop, (0), (n))
#define all(c) begin(c), end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9) + 7;
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
using pii = pair<int, int>;
// template<class T> ostream &operator<<(ostream &os,T &t){dump(t);return os;}
template <typename T, typename S>istream &operator>>(istream &is, pair<T, S> &p) { is >> p.first >> p.second; return is; }
//template <typename T, typename S>ostream &operator<<(ostream &os, pair<T, S> &p) {os << p.first << " " << p.second;return os;}

template <typename T> void printvv(const vector<vector<T>> &v) {
	cerr << endl;
	rep(i, 0, v.size()) rep(j, 0, v[i].size()) {
		if (typeid(v[i][j]).name() == typeid(INF).name() and v[i][j] == INF) {
			cerr << "INF";
		}
		else
			cerr << v[i][j];
		cerr << (j == v[i].size() - 1 ? '\n' : ' ');
	}
	cerr << endl;
}

#ifndef _DEBUG
#define printvv(...)
#endif
void YES(bool f) { cout << (f ? "YES" : "NO") << endl; }
void Yes(bool f) { cout << (f ? "Yes" : "No") << endl; }
template <class T, class U>bool chmin(T& a, U b) { if (a > b) { a = b; return true; }return false; }
template <class T, class U>bool chmax(T& a, U b) { if (a < b) { a = b; return true; }return false; }

using Weight = int;
using Flow = int;
struct Edge {
	int s, d; Weight w; Flow c;
	Edge() {};
	Edge(int s, int d, Weight w = 1) : s(s), d(d), w(w), c(w) {};
};
bool operator<(const Edge &e1, const Edge &e2) { return e1.w < e2.w; }
bool operator>(const Edge &e1, const Edge &e2) { return e2 < e1; }
inline ostream &operator<<(ostream &os, const Edge &e) { return (os << '(' << e.s << ", " << e.d << ", " << e.w << ')'); }

using Edges = vector<Edge>;
using Graph = vector<Edges>;
using Array = vector<Weight>;
using Matrix = vector<Array>;

void addArc(Graph &g, int s, int d, Weight w = 1) {
	g[s].emplace_back(s, d, w);
}
void addEdge(Graph &g, int a, int b, Weight w = 1) {
	addArc(g, a, b, w);
	addArc(g, b, a, w);
}

bool warshallFloyd(Matrix &dist) {
	int n = dist.size();
	for (int i = 0; i < n; i++) dist[i][i] = 0;
	for (int k = 0; k < n; k++)
		for (int i = 0; i < n; i++)
			for (int j = 0; j < n; j++) {
				if (dist[i][k] == INF || dist[k][j] == INF)continue;
				dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
			}
	bool negative_cycle = false;
	for (int i = 0; i < n; i++)
		if (dist[i][i] != 0)
			negative_cycle = true;
	return !negative_cycle;
}


signed main(signed argc, char *argv[]) {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(12);

	int N, K; cin >> N >> K;

	vector<vector<int>>m(N, vector<int>(N, 1));
	rep(i, 0, N) {
		m[i][i] = 0;
	}
	//printvv(m);
	// fix 0
	int x = 1, y = 2;
	int E = (N - 1) * (N - 2) / 2;
	if (K > E) {
		cout << -1 << endl;
		return 0;
	}
	loop(K) {
		m[x][y] = m[y][x] = INF;
		y++;
		if (y == N) {
			x++;
			y = x + 1;
		}
	}
	//printvv(m);
	warshallFloyd(m);
	//printvv(m);

	int cnt = 0;
	rep(i, 0, N) {
		rep(j, i + 1, N) {
			if (m[i][j] == 2) {
				cnt++;
			}
		}
	}
	assert(cnt == K);
	vector<pii>ans;
	rep(i, 0, N) {
		rep(j, i + 1, N) {
			if (m[i][j] == 1) {
				ans.eb(i, j);
			}
		}
	}
	cout << ans.size() << endl;
	for (auto p : ans) {
		cout << p.first + 1 << " " << p.second + 1 << endl;
	}
	return 0;
}


Measured memory_bytes: 640