#include<cstdio>
#include<algorithm>
using namespace std;
#define M 1000001
#define N 50001
struct P
{int x,y;};
P p[N];
int i,m,n,k,l,t=-M;
int C(P a,P b)
{return a.x<b.x||(a.x==b.x&&a.y<b.y);}
int main()
{FILE *f=fopen("orase.in","r"),*g=fopen("orase.out","w");
fscanf(f,"%d%d",&n,&m);
for(i=1;i<=m;i++)
       {fscanf(f,"%d%d",&p[i].x,&p[i].y);
       if(p[i].x+p[i].y>k)
             k=p[i].x+p[i].y,l=i;}
sort(p+1,p+m+1,C);
for(i=1;i<=m;i++)
if(l!=i&&p[i].y-p[i].x>t)
       t=p[i].y-p[i].x;
fprintf(g,"%d",k+t);
return 0;}