#include <stdio.h>

int m,n,l[1000001],d[1000001],rez,comp=0;



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",&d[i]);
         scanf("%d",&l[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;
             aux = l[i];
             l[i] = l[j];
             l[j] = aux;
             }
              for (  i = 2; i <= n; i++ )
    {
        if ( l[i] + l[comp] + (d[i] - d[comp]) > rez )
            rez = l[i] + l[comp] + (d[i]- d[comp]);

        if ( l[comp] + (m - d[comp]) < l[i] + (m - d[i]) )
            comp = i;
    }
             
               
}                 
int main() {
int i;
    freopen("orase.out","w",stdout);
    citire();
    rezolvare();
    printf("%d",rez);
    return 0;
}