#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
struct strada
{
    int st,lung;
};
bool cmp(strada x,strada y)
{
    if(x.st<y.st)
        return true;
    else
        return false;
}
strada v[1000001];
int main()
{
    ifstream fin ("orase.in");
    ofstream fout ("orase.out");
    int m,n,i,max=-2000000000,max2=-2000000000,poz,aux;
    fin>>m>>n;
    for(i=1; i<=n; i++)
        fin>>v[i].st>>v[i].lung;
    sort(v+1,v+n+1,cmp);
    for(i=1;i<=n;i++)
    {
        aux=v[i].st;
        v[i].st=v[i].lung-v[i].st;
        v[i].lung+=aux;
        if(v[i].st>max)
        {
            max=v[i].st;
            poz=i;
        }
    }
    for(i=1;i<poz;i++)
    {
        if(max2<v[i].lung)
            max2=v[i].lung;
    }
    for(i=poz+1;i<=n;i++)
         if(max2<v[i].lung)
            max2=v[i].lung;
            fout<<max+max2;
    return 0;
}