#include<iostream>
#include<fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");


int n,m,l,d,i;
int maximus(int x,int y){
	if(x<y)
		return y;
	else
		return x;
}
int main(){
	fin>>m>>n;
	int max1=0,max2=0;
	for(i=1;i<=n;++i){
		fin>>d>>l;
		max1=maximus(max1,d+l);
		max2=maximus(max2,l-d);
	}
	int rez=max1+max2;
	fout<<max1+max2;
}