#include<bits/stdc++.h>

using namespace std;

int n,m,ans;

const int N=1e6+;

int to[N];

struct node

{

    int to,nex;

}e[N];

int x,y,tot;

int head[N];

bool vis[N];

void add(int a,int b)

{

    e[++tot].to=b;

    e[tot].nex=head[a];

    head[a]=tot;

}

bool dfs(int x)

{

    for(int i=head[x];i;i=e[i].nex)

    {

        int xx=e[i].to;

        if(!vis[xx])

        {

            vis[xx]=;

            if(!to[xx]||dfs(to[xx]))

            {

                to[xx]=x;

                return ;

            }

        }

    }

    return ;

}

int main()

{

    cin>>n>>m;

    cin>>x>>y;

    while(x!=-&&y!=-)

    {

        if(x<=n&&y<=m) add(x,y);

        cin>>x;cin>>y;

    }

    for(int i=;i<=n;i++)

    {

        memset(vis,,sizeof(vis));

        if(dfs(i)) ans++;

    }

    cout<<ans<<endl;

    for(int i=n+;i<=m;i++)

    {

        if(to[i]) cout<<to[i]<<" "<<i<<endl;

    }

    return ;

}