#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;
const int N = 50000;
struct strada{
    int stanga,lung;

};
strada v[N+1];
ifstream fi("orase.in");
ofstream fo("orase.out");
bool cmp(strada a, strada b)
{
    if(a.stanga<b.stanga)
    return true;
    else
    return false;
}
int main()
{
    int i,j,m,n,s1=-1000,s2=-1000,smax,c,poz;
    fi>>m>>n;
    for(i=1;i<=n;i++)
    {
        fi>>v[i].stanga>>v[i].lung;
    }
    sort(v+1,v+n+1,cmp);
    for(i=1;i<=n;i++)
    {
       c=v[i].stanga;
       v[i].stanga=v[i].lung-v[i].stanga;
       v[i].lung+=c;
       if(v[i].stanga>s1){
            s1=v[i].stanga;
            poz=i;
       }
    }
       for(i=1;i<=n;i++)
       {
        if(v[i].lung>s2){
            s2=v[i].lung;

        }

       }
    smax=s1+s2;
    fo<<smax;

    return 0;
}