#include <bits/stdc++.h>
#define REP(i, l, r) for (int i = (l); i <= (r); ++ i)
#define DEP(i, r, l) for (int i = (r); i >= (l); -- i)
#define fi first
#define se second
#define pb emplace_back
#define mems(x, v) memset((x), (v), sizeof(x))
using namespace std;
namespace Milkcat {
	typedef long long LL;
	typedef pair<LL, LL> pii;
	const int N = 1005;
	int n, m, x, ct[N], in[N], rd[N], vs[N];
	vector<int> r[N][N], G[N]; vector<pii> p, rs;
	void dfs(int u) {
		vs[u] = 1;
		for (int &i = rd[u]; i < G[u].size(); ) {
			int x = G[u][i ++];
			dfs(x), p.pb(u, x);
		}
	}
	void solve(int s) {
		p.clear(), dfs(s);
		int x = p[0].se, y = p.back().se - n, t = r[x][y].back();
		exit(0);
		r[x][y].pop_back(), r[x][y].pb(n * m + 1), rs.pb(t, n * m + 1);
		for (int i = 0; i < p.size(); i += 2) {
			int x = p[i + 1].fi, y = p[i].fi - n, z = r[x][y].back();
			rs.pb(z, t), t = z, r[x][y].pop_back();
		}
	}
	int main() {
		cin >> n >> m;
		REP(i, 1, n) {
			REP(j, 1, m) ct[j] = 0;
			REP(j, 1, m) {
				cin >> x, ct[x] ++;
				if (ct[x] > 1) G[i].pb(x + n), in[x + n] ++, r[i][x].pb((i - 1) * m + j);
			}
			REP(j, 1, m)
				if (!ct[j]) G[j + n].pb(i), in[i] ++;
		}
		REP(i, 1, n) if (in[i] < G[i].size()) exit(-1);
		REP(i, 1, n) if (!vs[i]) solve(i);
		cout << rs.size() << '\n';
		for (auto [x, y] : rs)
			cout << x << ' ' << y << '\n';
		return 0;
	}
}
int main() {
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int T = 1;
	while (T --) Milkcat::main();
	return 0;
}