#include <bits/stdc++.h>
using namespace std;

using ll = long long;
#define endl '\n'
const int N = 1e2 + 10;
const int INF = 0x3f3f3f3f;

class Dinic {
private:
    struct Edge {
        int v, nxt, cap, flow;
    } e[N * N];
    int fir[N], cnt = 0;
    int dep[N], cur[N];
    ll maxflow = 0;
    int S, T;
public:
    void init (int s, int t) {
        S = s;
        T = t;
        memset(fir, -1, sizeof fir);
        cnt = 0;
    }
    void addEdge (int u, int v, int c) {
        e[cnt] = {v, fir[u], c, 0};
        fir[u] = cnt ++;
        e[cnt] = {u, fir[v], 0, 0};
        fir[v] = cnt ++;
    }
    int bfs () {
        queue<int> q;
        memset(dep, 0, sizeof dep);
        dep[S] = 1;
        q.push(S);
        while (!q.empty()) {
            int x = q.front();
            q.pop();
            for (int i = fir[x]; ~i; i = e[i].nxt) {
                int y = e[i].v;
                if (!dep[y] && e[i].cap > e[i].flow) {
                    dep[y] = dep[x] + 1;
                    q.push(y);
                }
            }
        }
        return dep[T];
    }
    int dfs (int x, int flow) {
        if (x == T || flow == 0) return flow;
        int res = 0;
        for (int i = cur[x]; ~i; i = e[i].nxt) {
            int y = e[i].v;
            if (dep[y] == dep[x] + 1) {
                int d = dfs(y, min(flow - res, e[i].cap - e[i].flow));
                res += d;
                e[i].flow += d;
                e[i ^ 1].flow -= d;
                if (res == flow) return res;
            }
        }
        return res;
    }
    int dinic () {
        while (bfs()) {
            memcpy(cur, fir, sizeof cur);
            maxflow += dfs(S, INF);
        }
        return maxflow;
    }
    void print() {
        for (int x = 1; x < S; x ++) {
            for (int i = cur[x]; ~i; i = e[i].nxt) {
                int y = e[i].v;
                if(y < S && e[i].flow > 0) {
                    cout << x << ' ' << y << endl;
                }
            }
        }
    }
};
Dinic dinic;

int n, m;

void solve() {
    cin >> m >> n;
    int s = n + 1, t = n + 2;
    dinic.init(n + 1, n + 2);
    for (int i = 1; i <= n; i ++) {
        if (i <= m) dinic.addEdge(s, i, 1);
        else dinic.addEdge(i, t, 1);
    }
    while (1) {
        int u, v;
        cin >> u >> v;
        if (u == - 1 && v == -1) break;
        dinic.addEdge(u, v, 1);
    }
    cout << dinic.dinic() << endl;
    dinic.print();
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    int _ = 1;
//  cin >> _;
    while (_--){
        solve();
    }
    return 0;
}