#include <stdio.h>
#include <algorithm>
using namespace std;
FILE *f=fopen("orase.in","r"),
    *g=fopen("orase.out","w");
struct orase{
    int l,d;
}a[50001];
int cmp(orase a, orase b){
    if(a.d<b.d)
        return 1;
   return 0;
}
int m,n,i,j,bun;
int main(){
        fscanf(f,"%d %d",&m,&n);
        for(i=1;i<=n;i++)
            fscanf(f,"%d %d",&a[i].d,&a[i].l);
        sort(a+1,a+n+1,cmp);
        j=1;
        bun=a[1].l-a[1].d;
        int max=0;
        for(i=2;i<=n;i++){
            if(a[i].l+a[i].d+bun>max)
               max=a[i].l+a[i].d+bun;
            if(a[i].l-a[i].d>bun)
                bun=a[i].l-a[i].d;
        }
        fprintf(g,"%d",max);
    return 0;
}