#include <stdio.h>
#define DIM 2001


struct inter {
  long int l;
  long int d;
};

typedef inter tip;

tip v[DIM];
int a[DIM];
int n,c,i,m,max;


void cre(tip *v, long int n);
void corect(long int poz, tip *v, long int n);
void sort(tip *v, long int n);

int main(){
  FILE *f = fopen("orase.in","r");
  fscanf(f,"%d %d",&m,&n);
  for (i=1;i<=n;i++)
    fscanf(f,"%d %d",&v[i].d,&v[i].l);
  fclose(f);
  sort(v,n);
  a[1]=v[1].d;
  for (i=2;i<=n;i++) {
    if (a[i-1]>v[i-1].l)
      a[i]=a[i-1];
    else
      a[i]=v[i-1].l;
    a[i]=a[i]+v[i].d-v[i-1].d;
  }

  max = 0;
  for (i=1;i<=n;i++)
    if (a[i]+v[i].l>max)
      max=a[i]+v[i].l;

  FILE *g = fopen("orase.out","w");
  fprintf(g,"%d",max);
  fclose(g);

  return 0;
}






void cre(tip *v, long int n){
  long int i,c,p;
  tip aux;
  for (i=2;i<=n;i++) {
    c = i;
    p = i>>1;
    while ((p)&&(v[c].d>v[p].d)) {
      aux = v[c];
      v[c] = v[p];
      v[p] = aux;
      c = p;
      p = p>>1;
    }
  }
}

void corect(long int poz, tip *v, long int n){
  long int p,c;
  tip aux;
  p = poz;
  c = p<<1;
  while (c<=n) {
    if ((c+1<=n) && (v[c+1].d>v[c].d))
      c++;
    if (v[c].d>v[p].d) {
      aux = v[c];
      v[c] = v[p];
      v[p] = aux;
      p = c;
      c = p<<1;
    } else break;
  }

}

void sort(tip *v, long int n) {
  long int i;
  tip aux;
  cre(v,n);
  for (i=n;i>1;i--) {
    aux = v[1];
    v[1] = v[i];
    v[i] = aux;
    corect(1,v,i-1);
  }
}