import java.util.Scanner;
public class Main {
public static class edg{
private int to;
private int next;
public edg() {
}
public edg(int to,int next) {
this.next=next;
this.to=to;
}
}
static int head[],t=0;
static edg a[];
public static void main(String[] args) {
Scanner sc=new Scanner(System.in);
int n=sc.nextInt();
int m=sc.nextInt();
a=new edg[n*5];
head=new int[n*5];
for (int i = 0; i<n*5; i++) {
head[i]=-1;
}
for (int i = 0; i <m; i++) {
int x=sc.nextInt();
int y=sc.nextInt();
add(x,y);
}
int f[]=new int[n+2];
for (int i =n; i>0; i--) {
if(head[i]==-1) {
f[i]+=f[i+1]+1;
}else {
for (int j =head[i]; j!=-1; j=a[j].next) {
int k=a[j].to;
if(i+k<=n) {
f[i]=Math.max(f[i],f[i+k]);
}
}
}
}
System.out.println(f[1]);
}
public static void add(int x,int y) {
a[t]=new edg(y,head[x]);
head[x]=t++;
}
}