// 112
#include <bits/stdc++.h>
using namespace std;
int n, d;
struct node
{
    int x, y;
    double xl, xr;
} is[1005];
int main()
{
    scanf("%d%d", &n, &d);
    for (int i = 1; i <= n; ++i)
    {
        scanf("%d%d", &is[i].x, &is[i].y);
        if (abs(is[i].y) > d)
        {
            puts("-1");
            return 0;
        }
        double len = sqrt(1.0 * d * d - 1.0 * is[i].y * is[i].y);
        is[i].xl = is[i].x - len, is[i].xr = is[i].x + len;
    }
    sort(is + 1, is + n + 1, [](const node &x, const node &y)
         { return x.xr < y.xr; });
    int ans = 0;
    double now = -DBL_MAX;
    const double eps = 1e-10;
    for (int i = 1; i <= n; ++i)
        if (now + eps < is[i].xl)
            now = is[i].xr, ++ans;
    printf("%d\n", ans);
    return 0;
}