#include <fstream>
#include <algorithm>

using namespace std;

ifstream fin ("orase.in");
ofstream fout ("orase.out");

const int Dim = 100001;
pair < long long , long long > O[Dim];
int n,m;
 
int main() {
	
	fin >> m >> n;
	for ( int i = 1; i <= n; ++i)
		fin >> O[i].first >> O[i].second;
	sort(O + 1,O + 1 + n);
	long long  ma = O[1].second - O[1].first;
	long long rez = 0;
	for ( int i = 2; i <= n; ++i) {
		rez = max(rez, O[i].first + O[i].second + ma);
		ma = max(ma, O[i].second - O[i].first);
		}
	fout << rez;
}