#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int i,j,m,n,s,max1;
struct distanta
{
int di,li;
}a[50009];

bool cmp(distanta a, distanta b)
{
    if(a.li==b.li)
      return a.li<b.li;
      else
      return a.di<b.di;
}

int main()
{
   f>>m>>n;
        for(i=1;i<=n;i++)
           f>>a[i].di>>a[i].li;

        sort(a+1,a+n+1,cmp);

      s=a[n].li;
    for(i=n-1;i>=1;i--)
    {
        s+=a[i+1].di-a[i].di ;
        max1=max(max1,s+a[i].li);
        s=max(s,a[i].li);
    }
    g<<max1;

    return 0;
}