#include<iostream>
#include<cstdlib>
#include<cstdio>
#include<algorithm>
using  namespace std;
struct nod
{
    int a,b;
}sz[25005];
bool operator < (const nod&xx,const nod&yy)
{
    return xx.a<yy.a;
}
int main()
{
    //freopen("a.in","r",stdin);
    //freopen("a.out","w",stdout);
    int n,t,i;
    //scanf("%d %d",&n,&t);
    cin>>n>>t;
	    for(i=1;i<=n;i++)
	    {
	        nod gogo;
	        int x,y;
	        scanf("%d %d",&x,&y);
	        gogo.a=x;
	        gogo.b=y;
	        sz[i]=gogo;
	    }
	    sort(sz+1,sz+n+1);
	    int s=0,e=0;
	    int cnt=0;
	    bool flag=0;
	    for(i=1;i<=n;i++)
	    {
	        if(i==1&&sz[i].a>1)
	        {
	            cout<<"-1";
	            flag=1;
	            break;
	        }
	        if(i==1&&sz[i].a==1)
	        {
	            cnt++;
	            s=1;
	            e=sz[i].b;
	            continue;
	        }
	        if(i==n&&sz[i].b<t)
	        {
	            cout<<"-1";
	            flag=1;
	            break;
	        }
	        if(sz[i].a>e+1)
	        {
	            cout<<"-1";
	            flag=1;
	            break;
	        }
	        if(sz[i].a<=s&&sz[i].b>e)
	        {
	            e=sz[i].b;
	            continue;
	        }
	        if(sz[i].a>s&&sz[i].b>e)
	        {
	            cnt++;
	            s=e+1;
	            e=sz[i].b;
	            continue;
	        }
	    }
	    if(!flag)
	        cout<<cnt;
    return 0;
}