#include<cstdio>
#include<algorithm>
#include<cstring>
#define Nmax 50005
using namespace std;
int m,n,i,j,s,sol;
struct nod
{
    int l;
    int d;
}v[Nmax];
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",&m,&n);
    for (i=1;i<=n;i++)
    scanf("%d %d",&v[i].d,&v[i].l);
    sort(v+1,v+n+1,cmp);
    s=-2000000000;
    for (i=1;i<=n;i++)
    {
        if (v[i].l+s+v[i].d>sol) sol=v[i].l+s+v[i].d;
        s=max(s,v[i].l-v[i].d);
    }
    printf("%d",sol);
    return 0;
}