[POJ 1328] Radar Installation

#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstdio>
using namespace std;
#define N 1010

struct P
{
    double x1,x2;
    bool operator < (const P &t)const
    {
        if(x2!=t.x2) return x2<t.x2;
        return x1>t.x1;
    }
}p[N];

int main()
{
    int n,r;
    int iCase=1;
    int end_flag;
    while(scanf("%d%d",&n,&r),n||r)
    {
        end_flag=0;
        for(int i=1;i<=n;i++)
        {
            double x,y;
            scanf("%lf%lf",&x,&y);
            if(y>r) end_flag=1;
            p[i].x1=x-sqrt(r*r-y*y);
            p[i].x2=x+sqrt(r*r-y*y);
        }
        printf("Case %d: ",iCase++);
        if(end_flag)
        {
            printf("-1\n");
            continue;
        }
        sort(p+1,p+n+1);
        int cnt=1;
        double tmp=p[1].x2;
        for(int i=2;i<=n;i++)
        {
            if(p[i].x1>tmp)
            {
                cnt++;
                tmp=p[i].x2;
            }
        }
        printf("%d\n",cnt);
    }
    return 0;
}