#include <fstream>

using namespace std;
ifstream fin("ctc.in");
ofstream fout("ctc.out");
int n, m, a[2][200005], start[100005], a1[2][200005], start1[100005], viz[100005], st[100005], vf, sol[200005], lg, nrc;
void dfp(int nod);
void dfm(int nod);
void kosajaru();
int main()
{
    int i, x, y;
    fin >> n >> m;
    for (i = 1; i <= m; i++) {
        fin >> x >> y;
        a[0][i] = y;
        a[1][i] = start[x];
        start[x] = i;
        a1[0][i] = x;
        a1[1][i] = start1[y];
        start1[y] = i;
    }
    kosajaru();

    return 0;
}
void dfp(int nod)
{
    int x;
    viz[nod] = 1;
    x = start[nod];
    while (x) {
        if (viz[a[0][x]] == 0)
            dfp(a[0][x]);
        x = a[1][x];
    }
    st[++vf] = nod;
}
void dfm(int nod)
{
    int x;
    viz[nod] = 2;
    sol[++lg] = nod;
    x = start1[nod];
    while (x) {
        if (viz[a1[0][x]] == 1)
            dfm(a1[0][x]);
        x = a1[1][x];
    }
}
void kosajaru()
{
    int i, nod;
    for (i = 1; i <= n; i++)
        if (viz[i] == 0)
            dfp(i);
    while (vf) {
        nod = st[vf];
        if (viz[nod] == 1) {
            nrc++;
            dfm(nod);
            sol[++lg] = -1;
        }
        vf--;
    }
    fout << nrc << "\n";
    for (i = 1; i <= lg; i++) {
        if (sol[i] == -1)
            fout << "\n";
        else
            fout << sol[i] << " ";
    }
}