#include<bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
#define N   209
#define M 40009
struct edge{int to,flow,nxt;}dir[M];
int n,m,x,y,cnt,ans,dep[N],head[N],opt[N];
bool vis[N];
queue<int>q;
bool bfs()
{
	memset(dep,0x3f,sizeof(dep));
	memset(vis,0,sizeof(vis));
	while(!q.empty()) q.pop();
	vis[0]=1,dep[0]=1,q.push(0);
	while(!q.empty())
	{
		int u=q.front();q.pop();
		vis[u]=0;
		for(int i=head[u];i!=-1;i=dir[i].nxt)
		{
			int v=dir[i].to;
			if(dep[v]>dep[u]+1&&dir[i].flow)
			{
				dep[v]=dep[u]+1;
				if(!vis[v]) vis[v]=1,q.push(v);
			}
		}
	}
	return dep[n+1]!=INF;
}
int dfs(int id,int flow)
{
	if(id==n+1) return flow;
	int pos,used=0;
	for(int i=head[id];i!=-1;i=dir[i].nxt)
	{
		int to=dir[i].to;
		if(dep[to]==dep[id]+1&&dir[i].flow)
		{
			pos=dfs(to,min(flow-used,dir[i].flow));
			if(pos!=0)
			{
				dir[i].flow-=pos;
				dir[i^1].flow+=pos;
				used+=pos;
				if(id!=0&&to!=n+1) opt[id]=to,opt[to]=id;
				if(used==flow) return used;
			}
		}
	}
	if(!used) dep[x]=INF;
	return used;
}
int main()
{
	scanf("%d%d",&m,&n),ans=0;
	memset(head,-1,sizeof(head));
	while(scanf("%d%d",&x,&y)!=EOF)
	{
		if(x==-1&&y==-1) break;
		dir[++cnt]={y,1,head[x]},head[x]=cnt;
		dir[++cnt]={x,0,head[y]},head[y]=cnt;
	}
	for(int i=1;i<=m;i++) 
	{
		dir[++cnt]={i,1,head[0]},head[0]=cnt;
		dir[++cnt]={0,0,head[i]},head[i]=cnt;
	}
	for(int i=m+1;i<=n;i++) 
	{
		dir[++cnt]={n+1,1,head[i]},head[i]=cnt;
		dir[++cnt]={i,0,head[n+1]},head[n+1]=cnt;
	}
	while(bfs()) ans+=dfs(0,INF);
	printf("%d\n",ans);
	for(int i=1;i<=m;i++)
	{
		if(i==opt[opt[i]]) printf("%d %d\n",i,opt[i]);
		else if(i==opt[opt[i]]) printf("%d %d\n",i,opt[i]);
	}
	return 0;
}

WA on #6 #7 #10 #11 63pts Dinic