#include<cstdio>
#include<algorithm>
using namespace std;
#define N 50001
typedef struct {
    int x,y;
}P;
P p[N];
int i,m,n,k,l,t=-1000001;
int C(P a,P b)
{
    return a.x<b.x||(a.x==b.x&&a.y<b.y);
}
int main()
{
    freopen("orase.in","r",stdin),freopen("orase.out","w",stdout),scanf("%d%d",&n,&m);
    for(i=1;i<=m;++i) {
        scanf("%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;
    printf("%d",k+t);
    return 0;
}