// iostream is too mainstream
#include <cstdio>
// bitch please
#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <cmath>
#include <iomanip>
#define dibs reserve
#define OVER9000 1234567890
#define ALL_THE(CAKE,LIE) for(auto LIE =CAKE.begin(); LIE != CAKE.end(); LIE++)
#define tisic 47
#define soclose 1e-6
#define chocolate win
// so much chocolate
#define patkan 9
#define ff first
#define ss second
#define abs(x) ((x < 0)?-(x):x)
#define uint unsigned int
using namespace std;
// mylittledoge

int main() {
	cin.sync_with_stdio(0);
	cin.tie(0);
	int N,K;
	cin >> K >> N;
	vector< pair<int,int> > V(N);
	for(int i =0; i < N; i++) {
		cin >> V[i].ff >> V[i].ss;
		if(V[i].ff > V[i].ss) swap(V[i].ff,V[i].ss);}
	sort(V.begin(),V.end());
	vector<int> B(30000,0);
	int X =0;
	for(int i =N-1; i >= 0; i--) {
		int x =0;
		for(int j =V[i].ff+10000; j <= V[i].ss+10000; j++) x +=B[j];
		for(int j =V[i].ff+10000; j <= V[i].ss+10000; j++) {
			if(x >= K) break;
			if(B[j] == 0) B[j]++, x++, X++;}
		}

	cout << X << "\n";
	for(int i =0; i < 30000; i++) if(B[i] == 1) cout << i-10000 << "\n";
	return 0;}

// look at my code
// my code is amazing