#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for (int i = a; i < b; i++)
using ll = long long;

int main(){
    int n, k;
	cin >> n >> k;
    vector<int> a(n), b, d, e(k);
    rep(i, 0, n) cin >> a[i];
    b = a;
    sort(b.begin(), b.end());
    vector<vector<int>> c(k), f;
    rep(i, 0, n) {
        int m = i % k;
        c[m].push_back(a[i]);
    }
    f = c;
    rep(i, 0, k) sort(c[i].begin(), c[i].end());
    rep(i, 0, n) {
        int m = i % k;
        d.push_back(c[m][e[m]]);
        e[m]++;
    }
    if(d != b) {
        cout << -1 << endl;
        return 0;
    }
    using P = pair<int, int>;
    vector<P> ans;
    rep(i, 0, k) {
        int sz = f[i].size();
        vector<int> x(sz);
        rep(j, 0, sz) x[j] = (f[i][j] - i) / k;
        rep(j, 0, sz) {
            if(x[j] == j) continue;
            int l = j, m = x[j];
            while(l < m) {
                ans.emplace_back(i + l * k, i + (l + 1) * k);
                swap(x[l], x[l + 1]);
                l++;
            }
        }
    }
    
    cout << ans.size() << endl;
    if(ans.size()) {
        for(P p : ans) cout << p.first + 1 << " " << p.second + 1 << endl;
    }
}