#include <iostream>
#include <vector>
#define len(v) (int)v.size()
using namespace std;

vector<vector<int>> g;
vector<bool> used;
vector<int> mt;

bool dfs(int v) {
    if (used[v])
        return false;
    used[v] = true;
    for (auto u : g[v]) {
        if (mt[u] == -1 || dfs(mt[u])) {
            mt[u] = v;
            return true;
        }
    }
    return false;
}

int main() {
    int n, m;
    cin >> n >> m;
    g.resize(n + 1);
    used.resize(n + 1, false);
    mt.resize(m + 1, -1);
    for (int v = 1; v <= n; ++v) {
        int u; cin >> u;
        while (u != 0) {
            g[v].push_back(u);
            cin >> u;
        }
    }

    int ans = 0;
    for (int v = 1; v <= n; ++v) {
        for (int i = 0; i < len(used); ++i)
            used[i] = false;
        if (dfs(v))
            ++ans;
    }

    cout << ans << "\n";
    for (int v = 1; v <= m; ++v) {
        if (mt[v] != -1) {
            cout << v << " " << mt[v] << "\n";
        }
    }
}