#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>

using namespace std;

const int N = 1e3+10;

int n,d;
struct Segment
{
    double l,r;
    bool operator<(Segment&t)   const
    {
        return r<t.r;
    }
}seg[N];

int main()
{
    scanf("%d%d", &n, &d);
    bool failed = false;
    for(int i=0;i<n;i++)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        if(y>d) failed = true;
        else
        {
            double len = sqrt(d*d-y*y);
            seg[i].l = x-len, seg[i].r=x+len;
        }
    }
    if(failed)
        puts("-1");
    else
    {
        sort(seg,seg+n);

        int cnt=0;
        double last=-1e20;
        for(int i=0;i<n;i++)
        {
            if(last<seg[i].l)
            {
                cnt++;
                last=seg[i].r;
            }
        }
        printf("%d\n",cnt);
    }

    return 0;
}