#include <bits/stdc++.h>

using namespace std;
using ll = long long ;
const ll inf=1000000000;

ll n,m;
const ll N=110,M=6000;
ll h[N],ne[M],e[M],f[M],idx,d[N],cur[N];

void add(ll u,ll v,ll w){
    e[idx]=v,ne[idx]=h[u],f[idx]=w,h[u]=idx++;
    e[idx]=u,ne[idx]=h[v],f[idx]=0,h[v]=idx++;
}

bool bfs(){
    queue <ll> q;
    q.push(0);
    memset(d,-1,sizeof d);
    d[0]=0;cur[0]=h[0];
    while(q.size()){
        ll t=q.front();
        q.pop();
        for(int i=h[t];i!=-1;i=ne[i]){
            int ver=e[i];
            if(d[ver]==-1 && f[i]){
                d[ver]=d[t]+1;
                cur[ver]=h[ver];
                if(ver==n+1) return true;
                q.push(ver);
            }
        }
    }
    return false;
}

ll find(ll u,ll limit){
    if(u==n+1) return limit;
    ll flow=0;
    for(int i=cur[u] ; i!=-1 && flow <limit ;i=ne[i]){
        cur[u]=i;
        int ver=e[i];
        if(d[ver]==d[u]+1 && f[i]){
            ll t=find(ver,min(f[i],limit-flow));
            if(t==0) d[ver]=-1;
            f[i]-=t;f[i^1]+=t;flow+=t;
        }
    }
    return flow;
}

int main(){
    cin>>m>>n;
    idx=0;
    memset(h,-1,sizeof h);
    ll u,v;
    while(cin>>u>>v){
        if(u==-1) break;
        add(u,v,1);
    }
    for(ll i=1;i<=m;i++){
        add(0,i,1);
    }
    for(ll i=m+1;i<=n;i++){
        add(i,n+1,1);
    }
    ll ans=0,flow;
    while(bfs()) while(flow = find(0,inf)) ans+=flow;
    cout<<ans<<endl;
    for(ll i=0;i<idx;i+=2){
        if(e[i]>m && e[i]<=n && !f[i]){
            cout<<e[i^1]<<' '<<e[i]<<endl;
        }
    }

    return 0;
}