#include <algorithm>
#include <cmath> 
#include <iostream>
#include <utility>

using namespace std;

int n;
double r;
pair<double, double> line[1145];
int res;

int main () {

	freopen ("monitor.in", "r", stdin);
	freopen ("monitor.out", "w", stdout);
	
	cin >> n >> r;
	int x, y;
	for (int i = 1; i <= n; i++) {
	    cin >> x >> y;
	    if (y > r) {
	        cout << -1 << endl;
	        return 0;
        }
        line[i].first = x + sqrt (r * r - y * y);
        line[i].second = x - sqrt (r * r - y * y);
    }
    sort (line + 1, line + 1 + n);
    
    double now = -1e9;
    for (int i = 1; i <= n; i++) {
        if (now < line[i].second) {
            now = line[i].first;
            res++;
        }
    }
    
    cout << res << endl;

	return 0;
}