#include<fstream>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int n,m,i,max1,maxd,sum,no,lg,sum2,dist;
struct pie
{
    int l,d;
}a[1000001];
int main()
{
    f>>m>>n;
    for(i=1;i<=n;i++)
    {
        f>>a[i].l>>a[i].d;
        if(maxd<a[i].d)
        {
            maxd=a[i].d;
            no=i;
            lg=a[i].l;
        }
    }
    sum=maxd;
    for(i=1;i<=n;i++)
    {
        if(i!=no)
        {
            sum2=a[i].d;
            dist=a[i].l-lg;
            if(dist<0)
            dist=dist*-1;
            sum2+=dist;
        }
        if(sum2>max1)
        max1=sum2;
        sum2=0;
    }
    g<<max1+sum;
    return 0;
}