能转移的最左是其左边完整区间的最右左端点，最右是能覆盖它的最左左端点-1
#pragma GCC optimize ("O3")
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>
#define N 200005
using namespace std;
int l[N],r[N],n,m,f[N],q[N];
int read(){
int a=0;char ch=getchar();
while(ch<'0'||ch>'9')ch=getchar();
while(ch>='0'&&ch<='9'){a=a*10+(ch^48);ch=getchar();}
return a;
}
int main(){
n=read(); m=read();
for(register int i=1;i<=n+1;++i)r[i]=i-1;
for(register int i=1,x,y;i<=m;++i){
x=read(); y=read();
r[y]=min(r[y],x-1);
l[y+1]=max(l[y+1],x);
}
for(register int i=2;i<=n+1;++i)
l[i]=max(l[i],l[i-1]);
for(register int i=n;i>=1;--i)
r[i]=min(r[i],r[i+1]);
/*for(int i=1;i<=n;i++)
printf("%d %d %d
",i,l[i],r[i]);*/
for(register int i=1,j=1,head=1,tail=1;i<=n+1;++i){
while(j<=r[i]&&j<=n){
if(f[j]==-1){j++;continue;}
while(head<=tail&&f[j]>f[q[tail]])--tail;
q[++tail]=j++;
}
while(head<=tail&&q[head]<l[i])++head;
if(head<=tail)f[i]=f[q[head]]+1;
else f[i]=-1;
//printf("%d %d
",i,f[i]);
}
if(f[n+1]==-1)printf("%d
",f[n+1]);
else printf("%d
",f[n+1]-1);
return 0;
}