#include <bits/stdc++.h>
#define MAX 50000
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int d[MAX+5],l[MAX+5];
int main()
{
    int n,m,ales,i,smax=0;
    fin>>m>>n;
    for(i=1;i<=n;i++)
    {
        fin>>d[i]>>l[i];
        if(smax<=d[i]+l[i])
        {
            ales=i;
            smax=d[i]+l[i];
        }
    }
    for(i=1;i<=n;i++)
    {
        if(i!=ales && abs(d[ales]-d[i])+l[ales]+l[i]>smax)
            smax=abs(d[ales]-d[i])+l[ales]+l[i];
    }
    fout<<smax;
    fin.close();
    fout.close();
    return 0;
}