#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
unsigned long long n,m,i,max1,maxd,sum2,dist;
struct pie
{
    int l,d;
}a[50001];
int cmp(pie a,pie b)
{
    if(a.d!=b.d)
        return a.d<b.d;
    else
        return a.l<b.l;
}
int main()
{
    f>>m>>n;
    for(i=1;i<=n;i++)
      f>>a[i].l>>a[i].d;
    sort(a+1,a+n+1,cmp);
    for(i=1;i<n;i++)
    {
        sum2=a[i].d;
        dist=a[i].l-a[n].l;
        if(dist<0)
          dist=dist*-1;
        sum2+=dist;
        if(sum2>max1)
          max1=sum2;
        sum2=0;
    }
    g<<max1+a[n].d<<"\n";
    return 0;
}