#include <cstdio>
#include <algorithm>
#define Dim 50002
#define INF 2000000001

using namespace std;
int nr, L, i, maxd = -INF, maxc;
struct nod
{
    int d;
    int l;
}v[Dim];
int cmp(const nod a, const nod b)
{
    return a.d < b.d;
}
int main()
{
    freopen("orase.in", "r", stdin);
    freopen("orase.out", "w", stdout);
    scanf("%d %d", &L, &nr);
    for(i = 1; i <= nr; ++ i)
        scanf("%d %d", &v[i].d, &v[i].l);
    sort(v + 1, v + nr + 1, cmp);
    for(i = 1; i <= nr; ++ i)
    {
        if(maxd < (maxc + v[i].l + v[i].d))
        maxd = maxc + v[i].l + v[i].d;
        if(maxc < (v[i].l - v[i].d))
        maxc = v[i].l - v[i].d;
    }
    printf("%d", maxd);

    return 0;
}