#include<stdio.h>
int m,n,i,max;
struct oras{int d,l;};
oras x,y,z;
int main()
{
FILE*f=fopen("orase.in","r");
FILE*g=fopen("orase.out","w");

fscanf(f,"%d %d",&m,&n);
fscanf(f,"%d %d %d %d",&x.d,&x.l,&y.d,&y.l);
if (x.d<y.d)max=x.l+y.l+ y.d-x.d;
    else max=x.l+y.l+ x.d-y.d;
for (i=3;i<=n;i++)
    {fscanf(f,"%d %d",&z.d,&z.l);
     if (z.d<x.d)
	  {if (max<x.l+z.l+x.d-z.d) max=x.l+z.l+x.d-z.d;}
	  else
	    {if (max<x.l+z.l+z.d-x.d) max=x.l+z.l+z.d-x.d;}
     if (z.d<y.d)
	 {if (max<y.l+z.l+y.d-z.d) max=y.l+z.l+y.d-z.d;}
	 else {if (max<y.l+z.l+z.d-y.d) max=y.l+z.l+z.d-y.d;}
     if (x.l<y.l) {if (z.l>x.l) x=z;}
	else {if (z.l>y.l) y=z;}
    }

fprintf(g,"%d",max);
fclose(f);
fclose(g);
return 0;
}