#include <fstream>
#include <cmath>
using namespace std;

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

struct o
{
    int d, l;
}a[50001];

int n, m, i, Max = -2147000000;

int main()
{
    f >> m >> n;
    for (i = 1; i <= n; i++)
        f >> a[i].d >> a[i].l;

    for (i = 1; i <= n; i++)
        for (int j = i+1; j <= n; j++)
            if (a[i].l+a[j].l + abs(a[i].d-a[j].d) > Max)
                Max = a[i].l+a[j].l + abs(a[i].d-a[j].d);

    g << Max;
    return 0;
}