#include<stdio.h>
#include<math.h>
#include<iostream.h>

FILE*fin=fopen("orase.in","r");
FILE*fout=fopen("orase.out","w");

long m,n,i,j,L[50001],D[50001];

int citire()
{fscanf(fin,"%ld %ld",&m,&n);
 for(i=1;i<=n;i++) fscanf(fin,"%ld %ld",&D[i],&L[i]); }

int ordonare()
{int ad=0; long var;
while(ad==0)
{ad=1;
for(i=1;i<n;i++) if(L[i]<L[i+1])
		 {ad=0; var=L[i]; L[i]=L[i+1]; L[i+1]=var;
			var=D[i]; D[i]=D[i+1]; D[i+1]=var;}
}}

main()
{citire();  ordonare();
		  long var,max=0;;
for(i=2;i<=n;i++)
{var=L[1]+L[i]+abs(D[1]-D[i]); if(var>max) max=var;}

fprintf(fout,"%ld",max);
}