#include<cstdio>
#include<algorithm>
using namespace std;
const int NMAX = 500005;
int m,n,i,best,sol;
struct oras{int d,l;}a[NMAX];
bool cmp(oras a,oras b) {return a.d<a.l;}
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(i=1;i<=n;i++) scanf("%d%d",&a[i].d,&a[i].l);
    sort(a+1,a+n+1,cmp); best=a[1].l-a[1].d;
    for(i=2;i<=n;i++)
    {
        sol=max(sol,a[i].l+a[i].d+best);
        best=max(best,a[i].l-a[i].d);
    }
    printf("%d\n",sol);
    return 0;
}