#include <fstream>
#define L 50003
using namespace std;

ifstream f("orase.in");
ofstream g("orase.out");

int m, n, d[L], l[L], mx, k, k2;
long long dist;
int main()
{
    f >> m >> n;
    for (int i = 1; i <= n; i++)
        f >> d[i] >> l[i];
    mx = 0;
    for (int i = 1; i <= n; i++){
        dist = d[i] + l[i];
        if (dist > mx){
            mx = dist;
            k = i;
        }
    }
    //g << k << " ";
    mx = 0;
    for (int i = 1; i <= n; i++){
        dist = l[i] + l[k] + max(d[i], d[k]) - min(d[i], d[k]);
        if (dist > mx && i != k){
            mx = dist;
            k2 = i;
        }
    }
    /*dist = l[k] + l[k2];
    if (d[k] > d[k2]) dist += d[k] - d[k2];
        else dist += d[k2] - d[k];*/
    g << mx;
    return 0;
}