#include<fstream>
#include<algorithm>
using namespace std;
int n, m, i, j, maxim, jmax, s;
pair<int, int>  v[50001];
ifstream fin("orase.in");
ofstream fout("orase.out");
int main(){
	fin>> m >> n;
	for(i = 1; i <= n; i++){
		fin>> v[i].first >> v[i].second;
	}
	sort(v + 1, v + n + 1);
	j = 1;
	jmax = v[1].second - v[1].first;
	for(i = 2; i <= n; i++){
		s = v[i].second + v[j].second + v[i].first - v[j].first;
		if(s > maxim){
			maxim = s;
		}
		if(v[i].second - v[i].first > jmax){
			jmax = v[i].second - v[i].first;
			j = i;
		}
	}
	fout<< maxim;
	return 0;
}