#include <stdio.h>
#include <stdlib.h>
int m,n,l[1000001],d[1000001],max,maxi,max2;
long int sol;


void citire() {
 freopen("orase.in","r",stdin);
 freopen("orase.out","w",stdout);    
     int i;
     scanf("%d",&m);
     scanf("%d",&n);
     for (i=1; i<=n; i++) {
         scanf("%d",&l[i]);
         scanf("%d",&d[i]);
      }
}  
void rezolvare() {
     int k;
     int i,j,aux;
     for (i=1; i<=n-1; i++)
         for (j=i; j <= n; j++) 
             if (d[i] > d[j]) {
             aux = d[i]; 
             d[i] = d[j];
             d[j] = aux;
             }
            for (i=1; i<=n; i++)
                if (abs(l[i] - d[i]) > max) 
                  max = abs(l[i] - d[i]);     
                if (l[i] + d[i]  > max2)
                   max2 = l[i] + d[i];
                         
}                 
int main() {
 freopen("orase.out","w",stdout);
    citire();
    rezolvare();
    printf("%ld",max+max2+10);
    system("pause");
    return 0;
}