#include<stdio.h>
using namespace std;
const int N = 100005, M = 200005;
int vf[M], urm[M], lst[N], nr, noduri[N], sol[2 * N], x[M], y[M];
bool viz[N];
void adauga (int x, int y)
{
    vf[++nr] = y;
    urm[nr] = lst[x];
    lst[x] = nr;
}
void dfs (int x2)
{
    viz[x2] = true;
    int p, y2;
    p = lst[x2];
    while (p != 0)
    {
        y2 = vf[p];
        if (viz[y2] == false)
            dfs (y2);
        p = urm[p];
    }
    noduri[++nr] = x2;
}
void curata (int n, int m)
{
    int i;
    for (i = 1; i <= m; i++)
        vf[i] = urm[i] = 0;
    for (i = 1; i <= n; i++)
        lst[i] = viz[i] = 0;
}
void dfs2 (int x2)
{
    viz[x2] = true;
    sol[++nr] = x2;
    int p, y2;
    p = lst[x2];
    while (p != 0)
    {
        y2 = vf[p];
        if (viz[y2] == false)
            dfs2 (y2);
        p = urm[p];
    }
}
int main ()
{
    FILE *in, *out;
    in = fopen ("ctc.in", "r");
    out = fopen ("ctc.out", "w");
    int n, m;
    fscanf (in, "%d%d", &n, &m);
    int i;
    for (i = 1; i <= m; i++)
    {
        fscanf (in, "%d%d", &x[i], &y[i]);
        adauga (x[i], y[i]);
    }
    nr = 0;
    for (i = 1; i <= n; i++)
        if (viz[i] == false)
            dfs (i);
    nr = 0;
    curata (n, m);
    for (i = 1; i <= m; i++)
        adauga (y[i], x[i]);
    nr = 0;
    int nrSol = 0;
    for (i = n; i >= 1; i--)
        if (viz[noduri[i]] == false)
        {
            dfs2(noduri[i]);
            sol[++nr] = -1;
            nrSol++;
        }
    fprintf (out, "%d\n", nrSol);
    for (i = 1; i <= nr; i++)
    {
        if (sol[i] != -1)
            fprintf (out, "%d ", sol[i]);
        else
            fprintf (out, "\n");
    }
    return 0;
}