/* ༼⌐■ل͟■༽ */
#include <fstream>
#include <utility>
#include <algorithm>

std::pair<int, int> v[50000];

int main() {
	std::ifstream fin("orase.in");
	std::ofstream fout("orase.out");
	int n, m, dist;

	fin >> n >> n;
	for (int i = 0; i < n; i++) {
		fin >> v[i].first >> v[i].second;
	}
	std::sort(v, v + n, std::greater< std::pair<int, int> >());

	m = v[0].first + v[0].second;
	dist = 0;
	for (int i = 1; i < n; i++) {
		dist = std::max(dist, m - v[i].first + v[i].second);
		m = std::max(m, v[i].first + v[i].second);
	}

	fout << dist;
	return 0;
}