#pragma GCC optimize("O3")
#pragma GCC target("avx2")
#include <iostream>
#include <vector>
#include <algorithm>
#include <bitset>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <queue>
#include <ctime>

using namespace std;

typedef long long ll;
typedef long double ld;
#define fastInp cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
const ll SIZE= 1e5 * 3 + 10, INF = 1e9 * 1e9 + 2, MOD = 1e9 + 7;

vector<vector<ll>> graph;
ll q;
vector<ll> vec;
bool used[SIZE];

struct edge {
	int c, bal, ind, to, fr = -1;
};

vector<edge> edges;

int n, m, nd;
int dfs(int v, int curMin = INT32_MAX) {
	used[v] = true;
	if (v == nd) {
		return curMin;
	}

	for (auto to : graph[v]) {
		if (edges[to].bal > 0 && !used[edges[to].to]) {
			int val = dfs(edges[to].to, min(curMin, edges[to].bal));
			if (val > 0) {
				curMin = min(val, curMin);
				edges[to].bal -= curMin;
				edges[to ^ 1].bal += curMin;
				return curMin;
			}
		}
	}

	return 0;
}

int main() {
	cin >> n >> m;

	nd = n + m + 1;
	graph.resize(n + m + 2);
	for (int i = 1; i <= n; i++) {
		int q;
		cin >> q;
		while (q != 0) {
			q += n;
			edges.push_back({ 1, 1, int(edges.size() - 1), q , i});
			edges.push_back({ 0, 0, int(edges.size() - 1), i});
			graph[i].push_back(edges.size() - 2);
			graph[q].push_back(edges.size() - 1);
			cin >> q;
		}
	}

	for (int i = 1; i <= n; i++) {
		edges.push_back({ 1, 1, int(edges.size() - 1), i });
		edges.push_back({ 0, 0, int(edges.size() - 1), 0 });
		graph[0].push_back(edges.size() - 2);
		graph[i].push_back(edges.size() - 1);
	}
	for (int i = n + 1; i <= n + m; i++) {
		edges.push_back({ 1, 1, int(edges.size() - 1), nd });
		edges.push_back({ 0, 0, int(edges.size() - 1), i });
		graph[i].push_back(edges.size() - 2);
		graph[nd].push_back(edges.size() - 1);
	}

	ll q = dfs(0);
	while (q != 0) {
		for (int i = 0; i <= nd; i++) used[i] = false;
		q = dfs(0);
	}

	vector<pair<ll, ll>> ans;
	for (int i = 0; i < edges.size(); i++) {
		if (edges[i].fr != -1 && edges[i].bal == 0) ans.push_back({ edges[i].fr, edges[i].to - n });
	}
	cout << ans.size() << "\n";
	for (int i = 0; i < ans.size(); i++) {
		cout << ans[i].first << " " << ans[i].second << "\n";
	}
	return 0;
}