#include<stdio.h>
#include<vector>
#include<algorithm>
using namespace std;
int N,M,x,y,index[100100],lowlink[100100],ind=0,nr=0,s[100100],roots[100100];
vector <int> a[100100],S,rez;

void strongconnect(int k)
{
    index[k]=ind;
    lowlink[k]=ind;
    ++ind;
    S.push_back(k);
    s[k]=1;
    int z=a[k].size();
    for(int i=0;i<z;++i)
    {
        if(index[a[k][i]]==-1)
        {
            strongconnect(a[k][i]);
            lowlink[k]=min(lowlink[k],lowlink[a[k][i]]);
        }
        else if (s[a[k][i]]==1)
        {
            lowlink[k]=min(lowlink[k],index[a[k][i]]);
        }
    }

    if(lowlink[k]==index[k])
    {
        roots[k]=1;
        ++nr;
        int w=S.back();
        rez.push_back(w);
        S.pop_back();
        s[w]=0;
        while(w!=k)
        {
            w=S.back();
            rez.push_back(w);
            S.pop_back();
            s[w]=0;
        }
    }
    return;
}

int main()
{
    freopen("ctc.in","r",stdin);
    freopen("ctc.out","w",stdout);
    scanf("%d%d",&N,&M);
    for(int i=1;i<=M;++i)
    {
        scanf("%d%d",&x,&y);
        a[x].push_back(y);
    }

    for(int i=1;i<=N;++i)
        index[i]=-1;

    for(int i=1;i<=N;++i)
        if(index[i]==-1)
            strongconnect(i);
    printf("%d\n",nr);

    int d=rez.size();

    for(int i=0;i<d;++i)
    {
        printf("%d ",rez[i]);
        if(roots[rez[i]]==1)
            printf("\n");
    }
    return 0;
}