#include <bits/stdc++.h>
using namespace std;
const int N=1e3+5;
struct node{double l,r;}g[N];
int main()
{
    int n,d;scanf("%d%d",&n,&d);
    for(int i=1;i<=n;i++)
    {
        double x,y;scanf("%lf%lf",&x,&y);
        if(y>d){puts("-1");return 0;}
        double dist=sqrt(d*d-y*y);
        g[i].l=x-dist;
        g[i].r=x+dist;
    }
    sort(g+1,g+n+1,[](node x,node y){return x.l<y.l;});
    int ans=0;
    double R=-1e9;
    for(int i=1;i<=n;i++)
        if(R<g[i].l)  ans++,R=g[i].r;
        else                R=min(R,g[i].r);
    printf("%d\n",ans);
    return 0;
}