#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n, k; cin >> n >> k;
  if (k < (n + 1) / 2) {
    cout << "-1\n";
    return 0;
  }
  vector<pair<int, int>> ans;
  int matching = 0;
  for (int i = 0; i < n;) {
    if (matching + (n - i) > k) {
      ans.emplace_back(i, i + 1);
      ans.emplace_back(i, i);
      ans.emplace_back(i + 1, i + 1);
      i += 2;
    } else {
      ans.emplace_back(i, i);
      ++i;
    }
    ++matching;
  }
  assert(matching == k);
  cout << ans.size() << '\n';
  for (const auto [a, b] : ans) {
    cout << a + 1 << ' ' << b + 1 << '\n';
  }
  return 0;
}