#include<stdio.h>
#include<algorithm>
using namespace std;
int sol,nr,i;
struct str
{

    int d,l;
};
str v[50004];
bool sortare(str a, str b)
{

    return a.d<b.d;
}
int main()
{
    int n,m;
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d%d",&v[i].d,&v[i].l);
    }
    sort(v+1,v+1+n,sortare);
    nr=1;
    for(i=2;i<=n;i++)
    {
       // printf("%d %d\n",v[i].l,v[i].d);
        if(v[nr].l+v[i].l+v[i].d-v[nr].d>sol)
            sol=v[nr].l+v[i].l+v[i].d-v[nr].d;
        if(v[i].l>v[nr].l+v[i].d-v[nr].d)
            nr=i;
    }
    printf("%d",sol);
    return 0;
}