//#include "pch.h"
#include <iostream>
#include <algorithm>
#include <fstream>
#include <vector>
#include <climits>
using namespace std;
ifstream f("orase.in");
ofstream o("orase.out");
int m, n;
vector<int>v(50005, 0);
int a, b;
int ddr;
int dsta;
int main()
{
	f >> n >> m;
	for (size_t i = 0; i < m; i++)
	{
		int st, dr;
		f >> st >> dr;
		if (st + dr > ddr)
		{
			ddr = st + dr;
		}
		if (dr - st > dsta)
		{
			dsta = dr - st;
		}

	}
	o << dsta + ddr;


}