#include <stdio.h>
#include <algorithm>
using namespace std;
int const N=50001;
int p;

struct hrt {
    int d,l;
};
hrt v[N];

bool cmp(hrt x, hrt y)
{
    return x.d < y.d;
}

int main()
{
    int m,n,i,u=1;
    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,cmp);
    for(i=2;i<=n;i++)
    {
        if(v[i].d-v[u].d+v[i].l+v[u].l>p)
            p=v[i].d-v[u].d+v[i].l+v[u].l;
        if(v[i].d-v[u].d+v[u].l<v[i].l)
            u=i;
    }
    printf("%d",p);
    return 0;
}