#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ld, ld> pd;
typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;

#define FOR(i, a, b) for (int i = a; i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i, a, b) for (int i = (b) - 1; i >= a; i--)
#define F0Rd(i, a) for (int i = (a) - 1; i >= 0; i--)

#define sz(x) (int) (x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define ub upper_bound
#define lb lower_bound
#define all(x) x.begin(), x.end()

const char nl = '\n';

void solve() {
	int N, K; cin >> N >> K;
	if (K == N || K == N-2) {
		cout << -1 << nl; return;
	}
	if (K == 0 && N <= 5) {
		cout << N << nl;
		F0R(i, N) {
			cout << i+1 << " " << (i+1)%N+1 << nl;
		}
		return;
	}
	if (K == 0 && N == 6) {
		cout << 7 << nl;
		vpi eds = {{1, 4}, {1, 5}, {1, 6}, {2, 3}, {2, 5}, {2, 6}, {3, 4}};
		for (auto &a : eds) {
			cout << a.f << " " << a.s << nl;
		}
		return;
	}
	vpi eds;
	F0R(i, N) {
		if (i != K) eds.pb({i, K});
	}
	FOR(i, K+1, N) {
		if ((i-K-1) % 2 == 0) {
			if (i == N-1) {
				eds.pb({i, i-1});
			} else eds.pb({i, i+1});
		}
	}

	cout << sz(eds) << nl;
	for (auto &a : eds) {
		cout << a.f+1 << " " << a.s+1 << nl;
	}
}
int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);
	int T; cin >> T;
	while(T--) {
		solve();
	}

}