#include<cstdio>
#include<algorithm>
#include<vector>
using namespace std;
struct qwer{int d,l;};
qwer v[50001];
bool comparare(qwer a,qwer b)
{
 if(a.d>b.d)
    return false;
 return true;
}
int main()
{
 int n,m,i,max,l1,d1;
 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,comparare);
 max=0;
 l1=v[1].l;
 d1=v[1].d;
 for(i=2;i<=n;i++)
    {
     if(l1+v[i].d-d1+v[i].l>max)
        {
         max=l1+v[i].d-d1+v[i].l;
        }
     if(l1+v[i].d-d1<v[i].l)
        {
         d1=v[i].d;
         l1=v[i].l;
        }
    }
 printf("%d\n",max);
return 0;
}