#include <fstream>
#include <iostream>
#include <vector>
using namespace std;

int main(){
	ifstream f("orase.in");
	ofstream g("orase.out");
	int m, n;
	f >> m >> n;
	if(n == 1){
		g << 0;
		return 0; }
	vector<int> len(m+1, 0);
	int rez = 0;
	for(int i = 0, d, l; i < n; ++i){
		f >> d >> l;
		rez = max(rez, len[d] + l);
		len[d] = max(len[d], l); }
	for(int i = 1, best = len[0]; i <= m; ++i){
		rez = max(rez, best + 1 + len[i]);
		best = max(len[i], best+1); }
	g << rez;
	return 0; }