#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],var1,var2;

int citire()
{fscanf(fin,"%ld %ld",&m,&n);
 for(i=1;i<=n;i++) {fscanf(fin,"%ld %ld",&var2,&var1); D[i]=var1-var2; L[i]=var1+var2;} }

int ordonare()
{int ad=0; long var;
while(ad==0)
{ad=1;
for(i=1;i<n;i++) if(D[i]<D[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();
		  long max=-500000;
for(i=1;i<=n;i++)
if(D[i]>max) {max=D[i]; var1=i;}

for(i=1;i<=n;i++)
if(var1!=i) if(L[i]>max) max=L[i];



fprintf(fout,"%ld",max+D[var1]);
}