//minamoto

 #include<iostream>

 #include<cstdio>

 #include<cstring>

 #include<algorithm>

 #include<queue>

 #define inf 0x3f3f3f3f

 using namespace std;

 #define getc() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)

 char buf[<<],*p1=buf,*p2=buf;

 inline int read(){

     #define num ch-'0'

     char ch;bool flag=;int res;

     while(!isdigit(ch=getc()))

     (ch=='-')&&(flag=true);

     for(res=num;isdigit(ch=getc());res=res*+num);

     (flag)&&(res=-res);

     #undef num

     return res;

 }

 char sr[<<],z[];int C=-,Z;

 inline void Ot(){fwrite(sr,,C+,stdout),C=-;}

 inline void print(int x,char ch){

     if(C><<)Ot();if(x<)sr[++C]=,x=-x;

     while(z[++Z]=x%+,x/=);

     while(sr[++C]=z[Z],--Z);sr[++C]=ch;

 }

 const int N=,M=;

 int n,m,e,u,v,maxflow,dep[N],mac[N];

 int head[N],Next[M],ver[M],edge[M];

 int tot=,cur[N],s,t;

 queue<int> q;

 inline void add(int u,int v,int e){

     ver[++tot]=v,Next[tot]=head[u],head[u]=tot,edge[tot]=e;

     ver[++tot]=u,Next[tot]=head[v],head[v]=tot,edge[tot]=;

 }

 bool bfs(){

     memset(dep,-,sizeof(dep));

     for(int i=;i<=n+;++i) cur[i]=head[i];

     dep[s]=,q.push(s);

     while(!q.empty()){

         int u=q.front();q.pop();

         for(int i=head[u];i;i=Next[i]){

             int v=ver[i];

             if(dep[v]<&&edge[i])

             dep[v]=dep[u]+,q.push(v);

         }

     }

     return ~dep[t];

 }

 int dfs(int u,int limit){

     if(!limit||u==t) return limit;

     int flow=,f;

     for(int i=cur[u];i;i=Next[i]){

         cur[u]=i;int v=ver[i];

         if(dep[v]==dep[u]+&&(f=dfs(v,min(limit,edge[i])))){

             flow+=f,limit-=f;

             edge[i]-=f,edge[i^]+=f,mac[u]=v;

             if(!limit) break;

         }

     }

     return flow;

 }

 inline void dinic(){

     while(bfs()) maxflow+=dfs(s,inf);

 }

 int main(){

     m=read(),n=read(),s=,t=n+;

     while(~(u=read())&&~(v=read())){

         add(u,v,inf);

     }

     for(int i=;i<=m;++i) add(s,i,);

     for(int i=m+;i<=n;++i) add(i,t,);

     dinic();

     print(maxflow,);

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

     if(mac[i]&&mac[i]!=t)

     print(i,),print(mac[i],);

     Ot();

     return ;

 }