1 #include <cstdio>
 2 #include <cstring>
 3 #include <algorithm>
 4 #include <queue>
 5 #include <bitset>
 6 #define inc(i,j,k) for(int i=j;i<=k;i++)
 7 #define maxn 1510
 8 using namespace std;
 9 
10 inline int read(){
11     char ch=getchar(); int f=1,x=0;
12     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1; ch=getchar();}
13     while(ch>=‘0‘&&ch<=‘9‘)x=x*10+ch-‘0‘,ch=getchar();
14     return f*x;
15 }
16 struct e{int f,t,n;}es[maxn*100],es2[maxn*100]; int g[maxn],tot;
17 void pe(int f,int t,int ess){es[ess]=(e){f,t,g[f]}; g[f]=ess;}
18 int n,l,topo[maxn],du[maxn];
19 bool cmp1(e a,e b){return topo[a.t]==topo[b.t]?topo[a.f]>topo[b.f]:topo[a.t]<topo[b.t];}
20 bool cmp2(e a,e b){return a.f==b.f?a.t<b.t:a.f<b.f;}
21 queue<int>q; bitset<maxn>f[maxn];
22 int main(){
23     n=read(); l=read(); inc(i,1,l){int x=read(),y=read(); pe(x,y,i); du[y]++;} inc(i,1,n)if(!du[i])q.push(i);
24     while(!q.empty()){
25         int x=q.front(); q.pop();
26         for(int i=g[x];i;i=es[i].n){
27             du[es[i].t]--; if(!du[es[i].t])q.push(es[i].t),topo[es[i].t]=topo[x]+1;
28         }
29     }
30     sort(es+1,es+l+1,cmp1); inc(i,1,n)f[i][i]=1;
31     inc(i,1,l){if(!f[es[i].t][es[i].f])es2[++tot]=es[i]; f[es[i].t]|=f[es[i].f];}
32     sort(es2+1,es2+tot+1,cmp2); printf("%d\n",tot);
33     inc(i,1,tot)printf("%d %d\n",es2[i].f,es2[i].t); return 0;
34 }