#include <bits/stdc++.h> using namespace std; vector<pair<int, int>> a; int main() { int n, k; cin >> n >> k; int d = __gcd(n, k); if (n % 2 == 0) { cout << -1 << endl; return 0; } if (d == 1) { vector<int> b(n); int cnt = 1; for (int i = 0; i < n; i++) { b[i] = cnt; cnt = (cnt + k) % n; } for (int i = 0; i < n - 1; i++) { a.emplace_back(b[i], b[i + 1]); } } cout << a.size() << endl; for (int i = 0; i < a.size(); i++) { cout << a[i].first << " " << a[i].second << "\n"; } return 0; }