#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n,m,l,d,i;
    int maxst = 0;
    int maxdr = 0;
    FILE * fp;
    fp = fopen("orase.in","r");
    fscanf(fp,"%i",&m);
    fscanf(fp,"%i",&n);
    for(i = 0;i<n;i++)
    {
        fscanf(fp,"%d",&d);
        fscanf(fp,"%d",&l);
        if(d+l > maxst)
        {
            maxst = d+l;
        }
        if(m-d+l>maxdr)
        {
            maxdr=m-d+l;
        }
    }
    fclose(fp);
    fp = fopen("orase.out","w");
    fprintf(fp,"%i",maxst+maxdr-m);
    fclose(fp);
    return 0;
}