#include<stdio.h>
#include<algorithm>
using namespace std;
struct da
{
    unsigned d,l;
}v[50003];
bool cm(da a,da b)
{
    return a.d<b.d;
}
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
unsigned m,n,i,p,x;
scanf("%u%u",&m,&n);
for(i=1;i<=n;i++)
    scanf("%u%u",&v[i].d,&v[i].l);
    sort(v+1,v+n+1,cm);
    p=1;
    m=0;
    for(i=2;i<=n;i++)
    {
        x=v[i].d-v[p].d+v[i].l+v[p].l;
        if(x>m)
            m=x;
        if(v[i].d+v[p].l<v[i].l+v[p].d)
            p=i;
    }
    printf("%u\n",m);
    return 0;
}