#include <stdio.h>
#include <vector>
#define Nmax 255+2
#define Mmax 7000+2
#define pb push_back

using namespace std;

vector< int > v[Nmax];
int L[Nmax],R[Nmax],use[Nmax],beg[Nmax],LL[Nmax];
int N,M;

int cupleaza(int i){
	vector< int >:: iterator it;
	use[i]=1;
	
	for(it=v[i].begin(); it!=v[i].end(); ++it)
		if( !R[*it] || ( !use[R[*it]] && cupleaza(R[*it]) )){
			L[i]=*it; R[*it]=i;
			return 1;
		}
	return 0;
}

int main(){
	int i,still,x,y,nr=0,wh;
	freopen("strazi.in","r",stdin);
	freopen("strazi.out","w",stdout);
	scanf("%d%d",&N,&M);
	for(i=1;i<=M;++i){
		scanf("%d%d",&x,&y);
		v[x].pb(y);
	}
	
	for( still=1; still; ){
		still=0;
		memset(use,0,sizeof(use));
		for(i=1;i<=N;++i)
			if( !L[i] && cupleaza(i) )
				still=1;
	}
	
	for(i=1;i<=N;++i) 
		if( ! L[i] ) ++nr;
	printf("%d\n",--nr);
	for(i=1;i<=N;++i)
		if( !L[i] ){
			beg[i]=i;
			while( R[beg[i]] ) beg[i]=R[beg[i]];
		}
	for(i=1;i<=N && nr;++i)
		if( !L[i] ){
			printf("%d ",i); wh=i;
			for(++i; L[i]; ) ++i;
			printf("%d\n",beg[i]);
			LL[wh]=beg[i];
			--i; --nr;
		}
	for(i=1;i<=N;++i)
		if( L[i] ) LL[i]=L[i];

	for(i=1; LL[i]; ){
		printf("%d ",i);
		i=LL[i];
	}
	printf("%d\n",i);
	
	fclose(stdin); fclose(stdout);
	return 0;
}