#include <stdio.h>
int m,n,l[1000001],d[1000001],max;
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 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 (l[i] + d[i] + l[j] - d[j] > sol)
                      sol = l[i] + d[i] + l[j] - d[j];
                              
}   
int main() {
 freopen("orase.out","w",stdout);
    citire();
    rezolvare();
    printf("%ld",sol);
    return 0;
}