#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<vector>
#include<map>
#include<queue>
#include<algorithm>
#define ll long long
#define rep(i,a,b) for(int i=a;i<=b;++i)
#define per(i,a,b) for(int i=b;i>=a;--i)
using namespace std;
inline ll read(){
    ll x=0,f=1;
    char c=getchar();
    while (c<'0' || c>'9'){
        if (c=='-')  f=-1;
        c=getchar();
    }
    while (c>='0' && c<='9'){
        x=x*10+c-'0';
         c=getchar();
    }
    return x*f;
}
inline void print(ll x){
	if(x < 0) putchar('-'), x = -x;
	if(x > 9) print(x / 10);
	putchar(x % 10 + '0');
	return ;
}
inline void pprint(ll x){
	print(x); puts("");
}
inline void aprint(ll x, char c){
	print(x); putchar(c);
}
const int N = 1000;
const int M = 1000;
ll d[N], q[N * M], hd, tl;
ll nxt[M << 1], to[M << 1], head[N], cnt = 1, c[M << 1], pre[N], cur[N];
void add(ll x, ll y, ll z)
{
	nxt[++cnt] = head[x];
	to[cnt] = y;
	c[cnt] = z;
	head[x] = cnt;
}
const ll inf = 1145141919810;
bool bfs(int st, int ed)
{
	memset(d, 0, sizeof(d));
	hd = tl = 0;
	q[++tl] = st;
	d[st] = 1;
	while(hd < tl)
	{
		int x = q[++hd];
		for(int i = head[x]; i; i = nxt[i])
		{
			int y = to[i];
			if(d[y] == 0 && c[i])
			{
				d[y] = d[x] + 1;
				q[++tl] = y;
				if(y == ed) return 1;
			}
		}
	}
	return 0;
}
ll dfs(int x, int ed, ll mf)// mf表示剩余的流量 
{
	if(x == ed) return mf;
	ll sum = 0;
	for(int i = cur[x]; i; i = nxt[i])
	{
		cur[x] = i;
		int y = to[i];
		if(d[y] == d[x] + 1 && c[i])
		{
			ll f = dfs(y, ed, min(c[i], mf));
			c[i] -= f;
			c[i ^ 1] += f;
			sum += f;
			mf -= f;
			if(mf == 0) break;
		}
	}
	if(sum == 0) d[x] = 0;//剪枝 
	return sum;//这个就是这个点的ans 
}
ll dinic(int st, int ed)
{
	ll flow = 0;
	while(bfs(st, ed))
	{
		memcpy(cur, head, sizeof(head));
		flow += dfs(st, ed, inf);
	}
	return flow;
}
int vis[N];
void check(int st, int ed){
	queue<int> qu;
	qu.push(ed);
	while(qu.size()){
		int x = qu.front(); qu.pop();
//		cout << x << endl;
		if(vis[x]) continue;
		vis[x] = 1;
		
		for(int i = head[x]; i; i = nxt[i]){
			int y = to[i];
			if(c[i] > 0 && (i & 1)){
				if(x != ed && y != st)
					cout << y << " " << x << endl;
				qu.push(y);
			}
		}
	}
}
int main(){
	ll n, m; cin >> m >> n;
	ll x, y;
	while(cin >> x >> y && x != -1){
		add(x, y, inf);
		add(y, x, 0);
	}
	for(int i = 1; i <= m; ++i){
		add(n + 2, i, 1);
		add(i, n + 2, 0);
	}
	for(int i = m + 1; i <= n; ++i){
		add(i, n + 3, 1);
		add(n + 3, i, 0);
	}
	cout << dinic(n + 2, n + 3) << endl;
//	check(n + 2, n + 3);
	for(int i = 1; i <= cnt; ++i){
		if(i % 2 == 0 && to[i ^ 1] != n + 2 && to[i] != n + 3 && c[i ^ 1] > 0){
			cout << to[i ^ 1] << " " << to[i] << endl;
		}
	}
	return 0;
}

一直在WA第五个点