#include <bits/stdc++.h>

using namespace std;
using ll = long long;
#define rep(i, s, t) for (ll i = s; i < (ll)(t); i++)
#define all(x) begin(x), end(x)
template <class T> bool chmin(T& x, T y) {
	return x > y ? (x = y, true) : false;
}
template <class T> bool chmax(T& x, T y) {
	return x < y ? (x = y, true) : false;
}

void solve() {
	int n, k;
	cin >> n >> k;
	vector<int> p(n);
	rep(i, 0, n) cin >> p[i], p[i]--;
	vector<vector<int>> v(k);
	rep(i, 0, n) v[i % k].push_back(p[i]);
	vector<pair<int, int>> ans;
	rep(i, 0, k) {
		int f = 0;
		for(int x : v[i]) if(x % k != i) f = 1;
		if(f) {
			cout << "-1\n";
			return;
		}
		int m = v[i].size();
		rep(lp, 0, m) rep(j, 0, m - 1) {
			if(v[i][j] > v[i][j + 1]) {
				swap(v[i][j], v[i][j + 1]);
				ans.push_back({j * k + i, j * k + i + k});
			}
		}
	}
	cout << ans.size() << '\n';
	for(auto [x, y] : ans) {
		cout << x + 1 << ' ' << y + 1 << '\n';
	}
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout << fixed << setprecision(15);
	int t = 1;
	// cin >> t;
	while (t--) solve();
}