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


struct str{
	int x,y;
};

int m,n,i,j;
str a[1000];
int dp[1000];

int main(){
	fin>>m>>n;
	for(i=1;i<=n;++i){
		fin>>a[i].x>>a[i].y;		
	}
	//sortez;
	for(i=1;i<=n;++i)
		for(j=i+1;j<=n;++j)
		{
			if(a[i].x>a[j].x)
			{
				str aux;
				aux=a[i];
				a[i]=a[j];
				a[j]=aux;
			}
				
		}
	dp[n]=a[n].y-a[n].x;
	for(i=n-1;i>0;--i)
	{
		int maxim=a[i].y-a[i].x;
		if(maxim>dp[n])
			dp[n]=maxim;
	}
	fout<<dp[n]+a[n].x+a[n].y;;
	
}