/* * Uva-10382-Watering Grass.cpp *　　特别注意精度。。（感觉这道题的判题有问题，开始怎么交都WA，，过两天什么都没改，再交就AC了 ， 汗。。 * * 详细代码注释： http://apps.hi.baidu.com/share/detail/24628400 * *  Created on: 2011-10-3 *      Author: */#include <cstdio>#include <cmath>#include <algorithm>using namespace std;

const int maxN = 10000 + 5;const double eps = 1E-7;

int n, l, w;struct SData{double x, y;};SData interval[maxN];

//精度inline int lfcmp(double x){if (x > -eps) return 1;else return -1;}

//按X值排序inline int cmp(const void *a,const void *b){return lfcmp( (((SData*)a)->x)-(((SData*)b)->x) );}

//区间覆盖-贪心void greedy(){int i = 0, ans = 0, j;double cover = 0, tmpMax;bool covered = false;while(i < n){        j = i;        tmpMax = cover;while(j<n && lfcmp(cover - (interval[j].x)) > 0){if(lfcmp((interval[j].y) - tmpMax) > 0)                tmpMax = interval[j].y;            j++;        }if(i == j) break;

        ans++;        cover = tmpMax;        i = j;

if(lfcmp(cover - l) > 0){            covered = true;break;        }    }if(covered) printf("%d\n", ans);else printf("-1\n");

}

int main(){while(scanf("%d %d %d", &n, &l, &w) != EOF){int p[maxN], r[maxN];for(int i=0; i<n; i++)            scanf("%d %d", &p[i], &r[i]);

for(int i=0; i<n; i++){//碰不到边界，不考虑            if(r[i] * 2 < w){                interval[i].x = 0;                interval[i].y = 0;continue;            }

double d = sqrt((double)r[i]*r[i] - (double)w*w/4);            interval[i].x = p[i] - d;            interval[i].y = p[i] + d;        }

        qsort(interval, n, sizeof(SData), cmp);

        greedy();    }

return 0;}