#include<bits/stdc++.h>

#define rep(i,x,y) for(register int i=x;i<=y;i++)

#define dec(i,x,y) for(register int i=x;i>=y;i--)

using namespace std;

const int M=;

const int N=;

const int inf=0x7fffffff;

inline int read(){

    int x=,f=;char ch=getchar();

    while(!isdigit(ch)){if(ch=='-')f=-;ch=getchar();}

    while(isdigit(ch)){x=(x<<)+(x<<)+(ch^);ch=getchar();}

    return x*f;}

int n,m,a[M],b[M],l[N],r[N],q[N],f[N],head,tail;

int main(){

    n=read(),m=read();

    rep(i,,n+) r[i]=i-;

    rep(i,,m){

        int x=read(),y=read();

        r[y]=min(r[y],x-);

        l[y+]=max(l[y+],x);

    }

    dec(i,n,) r[i]=min(r[i+],r[i]);

    rep(i,,n+) l[i]=max(l[i-],l[i]);

    int j=;head=;tail=;

    rep(i,,n+){

        for(;j<=r[i]&&j<=n;j++){// 补充元素

            if(f[j]==-) continue;

            while(head<=tail&&f[q[tail]]<f[j]) tail--;

            q[++tail]=j;

        }

        while(head<=tail&&q[head]<l[i]) head++;

        if(head<=tail) f[i]=f[q[head]]+(i!=n+?:);

        else f[i]=-;

    }

    printf("%d\n",f[n+]);return ;

}