#include <bits/stdc++.h>
using namespace std;
#include <atcoder/modint>
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using tlll = tuple<ll, ll, ll>;
constexpr ll INF = 1LL << 60;
template<class T> bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
template<class T> bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
ll safemod(ll A, ll M) {return (A % M + M) % M;}
ll divfloor(ll A, ll B) {if (B < 0) {return divfloor(-A, -B);} return (A - safemod(A, B)) / B;}
ll divceil(ll A, ll B) {if (B < 0) {return divceil(-A, -B);} return divfloor(A + B - 1, B);}
template<class T> void unique(vector<T> &V) {V.erase(unique(V.begin(), V.end()), V.end());}
template<class T> void sortunique(vector<T> &V) {sort(V.begin(), V.end()); V.erase(unique(V.begin(), V.end()), V.end());}
#define FINALANS(A) do {cout << (A) << '\n'; exit(0);} while (false)

int main()
{
  ll N, K;
  cin >> N >> K;

  if (2 * K < N)
    FINALANS(-1);

  vector<ll> A, B;
  for (ll i = 0; i < N - K; i++)
  {
    A.push_back(2 * i), B.push_back(2 * i + 1);
  }
  for (ll i = 0; i < N; i++)
  {
    A.push_back(i), B.push_back(i);
  }

  ll M = A.size();
  cout << M << endl;
  for (ll i = 0; i < M; i++)
  {
    cout << A.at(i) + 1 << " " << B.at(i) + 1 << endl;
  }
}