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

const int N=110, M=(N*N+2*N)*2, inf=1e9;
int ver[M],nxt[M],fl[M],head[2*N],tot=1;
int q[2*N],d[2*N],nh[2*N];
int n,m,S,T;

void add(int x,int y,int z){
    ver[++tot]=y,fl[tot]=z,nxt[tot]=head[x],head[x]=tot;
}

bool bfs(){
    memset(d,0,sizeof d);
    d[S]=1,q[0]=S,nh[S]=head[S];

    int hh=0,tt=1;
    while(hh<tt){
        int x=q[hh++];
        for(int i=head[x];i;i=nxt[i]){
            int y=ver[i];
            if(d[y] || !fl[i])continue;

            d[y]=d[x]+1, nh[y]=head[y];
            if(y==T)return true;
            q[tt++]=y;
        }
    }
    return false;
}

int find(int x,int limit){
    if(x==T)return limit;

    int flow=0;
    for(int i=nh[x];i && flow<limit;i=nxt[i]){
        nh[x]=i;
        int y=ver[i];
        if(d[y]!=d[x]+1 || !fl[i])continue;

        int t=find(y,min(fl[i],limit-flow));
        if(t)flow+=t, fl[i]-=t, fl[i^1]+=t;
        else d[y]=0;
    }
    return flow;
}

int dinic(){
    int res=0,flow;
    for(;bfs();)
        while(flow=find(S,inf))res+=flow;
    return res;
}

int main(){
    cin>>m>>n;
    int x,y;
    while(cin>>x>>y, x!=-1)
        add(x,m+y,1), add(m+y,x,0);
    S=0, T=n+m+1;
    for(int i=1;i<=m;i++)
        add(S,i,1), add(i,S,0);
    for(int i=1;i<=n;i++)
        add(m+i,T,1), add(T,m+i,0);
    cout<<dinic()<<endl;
    for(int i=2;i<=tot;i+=2){
        x=ver[i^1], y=ver[i];
        if(x<1 || x>m || y<=m || y==T)continue;
        if(fl[i])continue;
        cout<<x<<" "<<y-m<<endl;
    }
    return 0;
}