#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream ka("orase.in");
ofstream ki("orase.out");
int m,n,sol,maxx,x,y,distmax[1000005];
int maxim(int a,int b)
{
    if(a>=b)
    return a;
    return b;
}
int main()
{
    ka>>m>>n;
    for(int i=1;i<=n;i++)
    {
        ka>>x>>y;
        distmax[x]=maxim(distmax[x],y);
    }
    sol=-1000005;
    maxx=sol;
    for(int i=0;i<=1000004;i++)
    {
        if(distmax[i])
        {
            sol=maxim(sol,distmax[i]+i+maxx);
            maxx=maxim(maxx,distmax[i]-i);
        }
    }
    ki<<sol;
}