#include<cstdio>
#include<algorithm>
#include<cmath>
#define N 1005
#define db double
using namespace std;
int n,ans;db r,x,y,z;const db eps=1e-8;
struct node{db l,r;bool operator<(node y){return l>y.l;}}a[N];
int main()
{
    freopen("monitor.in","r",stdin);freopen("monitor.out","w",stdout);
    scanf("%d%lf",&n,&r);
    for(int i=1;i<=n;i++)
    {
        scanf("%lf%lf",&x,&y);
        if(y>r)return !printf("-1");
        z=sqrt(r*r-y*y);
        a[i]={x-z,x+z};
    }
    r=1e12;
    for(int i=1;i<=n;i++)
        if(a[i].r+eps<r)
            r=a[i].l,ans++;
    printf("%d",ans);
    return 0;
}