#include <bits/stdc++.h>

#define pb push_back
#define f first
#define s second

using namespace std;

typedef pair <int, int> pii;
typedef vector <int> vi;
typedef vector <pii> vpii;

const int maxn = 270;

vi g[maxn];
int tin[2][maxn], p[2][maxn], n, m, cnt, ap[2][maxn], atin[2][maxn], sz[2], mt[maxn];
bool used[maxn];
vpii ans;

bool try_kuhn (int v) {
    if (used[v]) return false;
    used[v] = true;
    for (int i=0; i < g[v].size(); ++i) {
        int to = g[v][i];
        if (mt[to] == -1 || try_kuhn(mt[to])) {
            mt[to] = v;
            return true;
        }
    }
    return false;
}

int main() {

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    cin >> n >> m;
    sz[0] = n, sz[1] = m;
    for (int i = 0; i < n; ++i) {
        int id = -1;
        while (true) {
            cin >> id;
            if (id == 0) break;
            --id;
            g[i].pb(id);
        }
    }

    for (int i = 0; i < m; ++i) mt[i] = -1;
    cnt = 0;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) used[j] = 0;
        try_kuhn(i);
    }

    for (int i = 0; i < m; ++i)
        if (mt[i] != -1) ans.pb({mt[i] + 1, i + 1});
    cout << ans.size() << endl;
    for (int i = 0; i < ans.size(); ++i) cout << ans[i].f << " " << ans[i].s << endl;

    return 0;
}