1 #include<queue>
  2 #include<cstdio>
  3 #include<cstring>
  4 #include<algorithm>
  5 const int oo=0x3f3f3f3f;
  6 struct pnt{
  7     int hd;
  8     int lyr;
  9     int now;
 10 }p[10001];
 11 struct ent{
 12     int twd;
 13     int lst;
 14     int vls;
 15 }e[100000];
 16 int n,m;
 17 int s,t;
 18 int cnt;
 19 std::queue<int>Q;
 20 void ade(int f,int t,int v)
 21 {
 22     cnt++;
 23     e[cnt].twd=t;
 24     e[cnt].vls=v;
 25     e[cnt].lst=p[f].hd;
 26     p[f].hd=cnt;
 27     return ;
 28 }
 29 bool Bfs(void)
 30 {
 31     while(!Q.empty())
 32         Q.pop();
 33     for(int i=1;i<=n+m+2;i++)
 34         p[i].lyr=0;
 35     p[s].lyr=1;
 36     Q.push(s);
 37     while(!Q.empty())
 38     {
 39         int x=Q.front();
 40         Q.pop();
 41         for(int i=p[x].hd;i;i=e[i].lst)
 42         {
 43             int to=e[i].twd;
 44             if(p[to].lyr==0&&e[i].vls>0)
 45             {
 46                 p[to].lyr=p[x].lyr+1;
 47                 if(to==t)
 48                     return true;
 49                 Q.push(to);
 50             }
 51         }
 52     }
 53     return false;
 54 }
 55 int Dfs(int x,int fll)
 56 {
 57     if(x==t)
 58         return fll;
 59     for(int &i=p[x].now;i;i=e[i].lst)
 60     {
 61         int to=e[i].twd;
 62         if(p[to].lyr==p[x].lyr+1&&e[i].vls>0)
 63         {
 64             int ans=Dfs(to,std::min(fll,e[i].vls));
 65             if(ans>0)
 66             {
 67                 e[i].vls-=ans;
 68                 e[((i-1)^1)+1].vls+=ans;
 69                 return ans;
 70             }
 71         }
 72     }
 73     return 0;
 74 }
 75 int Dinic(void)
 76 {
 77     int ans=0;
 78     while(Bfs())
 79     {
 80         for(int i=1;i<=n+m+2;i++)
 81             p[i].now=p[i].hd;
 82         int dlt;
 83         while(dlt=Dfs(s,oo))
 84             ans+=dlt;
 85     }
 86     return ans;
 87 }
 88 int main()
 89 {
 90     //freopen("a.in","r",stdin);
 91     scanf("%d%d",&n,&m);
 92     s=n+m+1,t=s+1;
 93     for(int i=1;i<=n;i++)ade(s,i,1),ade(i,s,0);
 94     for(int i=1;i<=m;i++)ade(i+n,t,1),ade(t,i+n,0);
 95     while(true)
 96     {
 97         int a,b;
 98         scanf("%d%d",&a,&b);
 99         if(a==-1&&b==-1)
100             break;
101         ade(a,b,1);
102         ade(b,a,0);
103     }
104     int ans=Dinic();
105     if(ans)
106     {
107         printf("%d\n",ans);
108         for(int i=1;i<=n;i++)
109         {
110             for(int j=p[i].hd;j;j=e[j].lst)
111             {
112                 int to=e[j].twd;
113                 if(e[j].vls==0&&to>n&&to<=n+m)
114                 {
115                     printf("%d %d\n",i,to);
116                 }
117             }
118         }
119     }else
120         puts("No Solution!");
121     return 0;
122 }