#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.1,sse4.2,popcnt,abm,mmx,avx")
// #pragma comment(linker, "/stack:200000000"]

#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <unordered_set>
#include <unordered_map>
#include <set>
#include <map>
#include <queue>
#include <deque>
#include <bitset>
#include <stack>
#include <random>
#include <fstream>
#include <sstream>
#include <chrono>

#define fi first
#define se second
#define pb push_back
#define ll long long
#define ld long double
#define hm unordered_map
#define pii pair<int, int>
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define cinv(v) for (auto& x: v) cin >> x
#define fr(i, n) for (int i = 0; i < n; ++i)
#define fl(i, l, n) for (int i = l; i < n; ++i)

#define int ll

using namespace std;

#ifdef __LOCAL
	#define dbg(x) cerr << #x << " : " << x << '\n'
	const int maxn = 500 + 20;
#else
	#define dbg(x)
	const int maxn = 500 + 20;
#endif

//tg: @galebickosikasa

const ll inf = (ll) 2e9;
const ld pi = asin (1) * 2;
const ld eps = 1e-8;
const ll mod = (ll)1e9 + 7;
const ll ns = 97;
const int C = 250;

mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());

struct edge {
	int to, c, f;
};

vector<edge> edges;
vector<int> g[maxn];
int it[maxn], d[maxn];

int bfs (int s, int t) {
	for (auto& x: d) x = inf;
	d[s] = 0;
	queue<int> a;
	a.push (s);
	while (!a.empty ()) {
		int v = a.front ();
		a.pop ();
		for (auto& u: g[v]) {
			auto& e = edges[u];
			if (d[e.to] == inf && e.c - e.f > 0) {
				d[e.to] = d[v] + 1;
				a.push (e.to);
			}
		}
	}
	return d[t] < inf;
}

int dfs (int v, int t, int c = inf, int p = -1) {
	if (v == t) return c;
	while (it[v] < sz (g[v])) {
		auto& e = edges[g[v][it[v]]];
		if (e.to == p || e.c - e.f == 0 || d[v] + 1 != d[e.to]) {
			++it[v];
			continue;
		}
		int delta = dfs (e.to, t, min (c, e.c - e.f), v);
		if (delta > 0) {
			auto& _e = edges[g[v][it[v]]^1];
			e.f += delta;
			_e.f -= delta;
			return delta;
		}
		++it[v];
	}
	return 0;
}

signed main () {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	int n, m;
	cin >> n >> m;
	fr (i, n) {
		while (1) {
			int x;
			cin >> x;
			if (!x) break;
			--x;
			x += C;
			g[i].pb (sz (edges));
			edges.pb ({x, 1, 0});
			g[x].pb (sz (edges));
			edges.pb ({i, 0, 0});
		}
	}
	int s = 500, t = 501;
	fr (i, n) {
		g[s].pb (sz (edges));
		edges.pb ({i, 1, 0});
		g[i].pb (sz (edges));
		edges.pb ({s, 0, 0});
	}
	fr (i, m) {
		g[i + C].pb (sz (edges));
		edges.pb ({t, 1, 0});
		g[t].pb (sz (edges));
		edges.pb ({i + C, 0, 0});
	}

	while (bfs (s, t)) {
		while (dfs (s, t)) {}
		for (auto& y: it) y = 0;
	}

	vector<pii> ans;
	fr (i, n) {
		for (auto& u: g[i]) {
			auto& e = edges[u];
			if ((u & 1) == 0 && e.f) ans.pb ({i, e.to - C});
		}
	}

	cout << sz (ans) << '\n';
	for (auto& x: ans) {
		cout << x.fi + 1 << ' ' << x.se + 1 << '\n';
	}









}