#include <iostream>
#include <stack>
#include <vector>
#include <utility>

using namespace std;

vector<int> to_print;
int n, m, idx, cnt;
vector<pair<bool, vector<int>>> adj[2];	// 0 - graf normal, 1 graf intors
stack<int> s;

void dfs(int which, int nod){
	adj[which][nod].first = true;
	for(int i = 0; i < adj[which][nod].second.size(); i++){
		if(!adj[which][adj[which][nod].second.at(i)].first){
			if(which)
				to_print.push_back(adj[which][nod].second.at(i) + 1);
			dfs(which, adj[which][nod].second.at(i));
		}
	}
	if(!which)
		s.push(nod);
}

int main(){
	int a, b;
	freopen("ctc.in", "r", stdin);
	freopen("ctc.out", "w", stdout);
	
	scanf("%d%d", &n, &m);

	vector<int> v;
	for(int i = 0; i < n; i++){
		adj[0].push_back(make_pair(false, v));
		adj[1].push_back(make_pair(false, v));
	}

	for(int i = 0; i < m; i++){
		scanf("%d%d", &a, &b);
		adj[0][--a].second.push_back(--b);
		adj[1][b].second.push_back(a);
	}

	for(int i = 0; i < n; i++){
		if(!adj[0][i].first){
			dfs(0, i);
		}
	}

	while(!s.empty()){
		int nod = s.top();
		s.pop();
		if(!adj[1][nod].first){
			dfs(1, nod);
			to_print.push_back(nod+1);
			to_print.push_back(0);
			cnt++;
		}
	}
	printf("%d\n", cnt);
	for(int i = 0; i < to_print.size(); i++){
		if(!to_print[i])
			printf("\n");
		else
			printf("%d ", to_print[i]);
	}
	return 0;
}