#define _USE_MATH_DEFINES
#define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
#define _SILENCE_ALL_CXX17_DEPRECATION_WARNINGS
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
#include<iostream>
#include<unordered_set>

using namespace std;

#define dprint(Exp,...) if(Exp){fprintf(stderr, __VA_ARGS__);}
#define printe(...) fprintf(stderr, __VA_ARGS__);
//#define printe(...) void();
#define printE(...) fprintf(stderr, __VA_ARGS__);
//#define printE(...) void();


#define PrtExp(_Exp)  cerr<< #_Exp <<" = "<< (_Exp)
#define PrtExpN(_Exp)  cerr<< #_Exp <<" = "<< (_Exp) <<"\n"

#define SINT(n) scanf("%d",&n)
#define SINT2(n,m) scanf("%d %d",&n,&m)
#define SINT3(n,m,o) scanf("%d %d %d",&n,&m,&o)
#define SINT4(n,m,o,P) scanf("%d %d %d %d",&n,&m,&o,&P)
#define SINT5(n,m,o,P,q) scanf("%d %d %d %d %d",&n,&m,&o,&P,&q)
#define SLL(n) scanf("%lld",&n)
#define SLL2(n,m) scanf("%lld %lld",&n,&m)
#define SLL3(n,m,o) scanf("%lld %lld %lld",&n,&m,&o)
#define SST(s) scanf("%s",s)
#define SCH(c) scanf("%c",&c)

#define GC() getchar()

#define PINT(n) printf("%d",(int)(n))
#define PINT2(n,m) printf("%d %d",(int)(n),(int)(m))
#define PINT3(n,m,l) printf("%d %d %d",(int)(n),(int)(m),(int)(l))
#define PLL(n) printf("%lld",(long long)(n))
#define PST(s) printf("%s",(s))
#define PCH(s) printf("%c",(s))

#define PINTN(n) printf("%d\n",(int)(n))
#define PINT2N(n,m) printf("%d %d\n",(int)(n),(int)(m))
#define PINT3N(n,m,l) printf("%d %d %d\n",(int)(n),(int)(m),(int)(l))
#define PLLN(n) printf("%lld\n",(long long)(n))
#define PSTN(s) printf("%s\n",(s))
#define PCHN(s) printf("%c\n",(s))

#define PSP() printf(" ")
#define PN() printf("\n")

#define PC(c) putchar(c)
#define CSP (' ')
#define SN ("\n")

#define rep(i,a) for(int i=0;i<a;i++)
#define reP(i,a) for(int i=0;i<=a;i++)
#define Rep(i,a) for(int i=a-1;i>=0;i--)
#define ReP(i,a) for(int i=a;i>=0;i--)

#define rEp(i,a) for(i=0;i<a;i++)
#define rEP(i,a) for(i=0;i<=a;i++)
#define REp(i,a) for(i=a-1;i>=0;i--)
#define REP(i,a) for(i=a;i>=0;i--)

#define repft(i,a,b) for(int i=a;i<b;i++)
#define repfT(i,a,b) for(int i=a;i<=b;i++)
#define Repft(i,a,b) for(int i=a-1;i>=b;i--)
#define RepfT(i,a,b) for(int i=a;i>=b;i--)

#define foreach(a,it) for(auto it = a.begin(); it != a.end(); ++it)

#define FILL(a,v) fill(begin(a),end(a), v)
#define FILL0(a) memset(a,0,sizeof(a))
#define FILL1(a) memset(a,-1,sizeof(a))

typedef long long ll;

typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll>   Pll;


const int INF = 1'010'000'000; // 0x3C33'6080
const ll INFLL = 0x1f1f1f1f1f1f1f1fLL;//2,242,545,357,980,376,863

template <class A, class B> inline ostream& operator<<(ostream& st, const pair<A, B>& P) { return st << "(" << P.first << "," << P.second << ")"; };
template <class A, class B> inline pair<A, B> operator+(const pair<A, B>& P, const pair<A, B>& Q) { return pair<A, B>(P.first + Q.first, P.second + Q.second); };
template <class A, class B> inline pair<A, B> operator-(const pair<A, B>& P, const pair<A, B>& Q) { return pair<A, B>(P.first - Q.first, P.second - Q.second); };

#define fs  first
#define sc  second

int bitCount(unsigned int bits) {
	bits = (bits & 0x55555555) + (bits >> 1 & 0x55555555);    //  2bitごとに計算
	bits = (bits & 0x33333333) + (bits >> 2 & 0x33333333);    //  4bitごとに計算
	bits = (bits & 0x0f0f0f0f) + (bits >> 4 & 0x0f0f0f0f);    //  8bitごとに計算
	bits = (bits & 0x00ff00ff) + (bits >> 8 & 0x00ff00ff);    //  16ビットごとに計算   
	return (bits & 0x0000ffff) + (bits >> 16);    //  32ビット分を計算   
}


int g[102][128];


void hoge(int n) {
	vector<Pi> pa;
	rep(i, n) repft(j, i + 1, n) {
		pa.push_back({i,j});
	}
	vector<int> CC(n*n, 0);

	for (int s = (1 << pa.size()), f = 0; f < s; ++f) {
		rep(i, n) rep(j, n) g[i][j] = (i==j ? 0 : INF);

		rep(i, pa.size()) {
			int x = pa[i].fs;
			int y = pa[i].sc;
			if ((f >> i) & 1) {
				g[x][y] = g[y][x] = 1;
			}
		}
		rep(k, n) rep(i, n) rep(j, n) {
			g[j][i] = g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
		}
		bool ng = false;
		int cnt = 0;
		rep(i, n) repft(j, i+1, n) {
			if (g[i][j] == INF) ng = true;
			if (g[i][j] == 2) cnt++;
		}
		if (ng) continue;
		if (CC[cnt] == 0) {
			CC[cnt] = 1;
			cout << "cnt : " << cnt << endl;

			FILL0(g);
			rep(i, pa.size()) {
				int x = pa[i].fs;
				int y = pa[i].sc;
				if ((f >> i) & 1) {
					g[x][y] = g[y][x] = 1;
					cout << x << " " << y << endl;
				}
			}
			rep(i, n) {
				rep(j, n) {
					cout << g[i][j] << " ";
				}
				cout << endl;
			}
			cout << endl;
		}
	}
	cout << "FIN" << endl;
}

int fuga(int n) {
	for (;;) {
		rep(x, n) rep(y, n) {
			cin >> g[x][y];
			if (g[x][y] == 0 && x != y) g[x][y] = INF;
		}
		bool NG = false;
		rep(x, n) rep(y, n) {
			if (x == y) {
				if (g[x][y] != 0) NG = true;
			} else {
				if(g[x][y] != g[y][x]) NG = true;
			}
		}
		if (NG) return 0;

		rep(k, n) rep(i, n) rep(j, n) {
			g[j][i] = g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
		}
		bool ng = false;
		int cnt = 0;
		rep(i, n) repft(j, i + 1, n) {
			if (g[i][j] == INF) ng = true;
			if (g[i][j] == 2) cnt++;
		}
		if (ng) continue;
		cout << "cnt : " << cnt << endl;
	}
}

int main() {
	// hoge(8);
	// fuga(7);
	int n, k;
	cin >> n >> k;

	vector<Pi> pa;
	repft(i, 1, n) repft(j, i + 1, n) {
		pa.push_back({ i,j });
	}

	if (k > pa.size()) {
		cout << -1 << endl;
		return 0;
	}

	cout << pa.size() - k + n - 1 << endl;
	repft(i, 1, n) {
		cout << 1 << " " << i+1 << endl;
	}
	rep(i, pa.size() - k) {
		int x = pa[i].fs;
		int y = pa[i].sc;
		cout << x + 1 << " " << y + 1 << endl;
	}


	/*
	rep(i, n) {
		int x = pa.back().fs;
		int y = pa.back().sc;
		g[x][y] = 0;
	}
	*/






}




Measured memory_bytes: 384