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


struct str{
	int x,y;
};
int n,m,i,j;
str s[100];
int main(){
	fin>>m>>n;
	for(i=1;i<=n;++i){
		fin>>s[i].x>>s[i].y;
		
	}
	//ordonam crescator
	for(i=1;i<n;++i)
		for(j=i+1;j<=n;++j){
			if(s[i].x>s[j].x){
				str aux;
				aux=s[i];
				s[i]=s[j];
				s[j]=aux;
			}
		}
	int max=s[1].y,solutie=0;
	for(i=2;i<=n;++i){
		max+=s[i].x-s[i-1].x;
		if(s[i].y+max>solutie)
			solutie=s[i].y+max;
		if(s[i].y>max)
			max=s[i].y;		
	}
	fout<<solutie;
	
}