#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int m, n, i;
long long Max, lmax=-1;
struct oras {
    int d;
    int l;
}o[50001];
bool cmp(const oras &o1, const oras &o2)
{
    return o1.d < o2.d;
}
int main()
{
    f>>m>>n;
    for (i=1; i<=n; i++)
        f>>o[i].d>>o[i].l;
    sort (o+1, o+n+1, cmp);
    Max=o[1].l-o[1].d;
    for (i=2; i<=n; i++) {
        if (o[i].d+o[i].l+Max > lmax)
            lmax=o[i].d+o[i].l+Max;
        if (o[i].l-o[i].d > Max)
            Max=o[i].l-o[i].d;
    }
    g<<lmax;
    return 0;
}