#include <stdio.h>#include <math.h>#define eps 0.0000000001typedef struct s_point { double x, y;} Point;typedef struct s_line { // line: a*x + b*y + c = 0 double a, b, c;} Line;int equal(double x, double y) { if (fabs(x - y) < eps) { return 1; } return 0;}// calculate the perpendicular bisector of line defined by p1 and p2Line bisector(Point p1, Point p2) { Line line; if (equal(p1.x, p2.x)) { line.a = 0.0, line.b = 1.0, line.c = - (p1.y + p2.y) / 2.0; } else if (equal(p1.y, p2.y)) { line.a = 1.0, line.b = 0.0, line.c = - (p1.x + p2.x) / 2.0; } else { double k = - (p2.x - p1.x) / (p2.y - p1.y); double cx = (p1.x + p2.x) / 2.0; double cy = (p1.y + p2.y) / 2.0; double b = cy - k * cx; line.a = k, line.b = -1, line.c = b; } return line;}Point ps[200];int n = 0, max = 1;// count the number of points in the circle whose center is cint count(Point c) { int i, ret = 0; for (i = 0; i < n; i++) { double dx = ps[i].x - c.x; double dy = ps[i].y - c.y; double d = sqrt(dx*dx + dy*dy); if (d > 2.5) continue; // the radius equals 2.5 ret++; } return ret;}int main() { double x, y; int i, j; while (scanf("%lf%lf", &x, &y) != EOF) { ps[n].x = x, ps[n].y = y; n++; } for (i = 0; i < n; i++) { for (j = i+1; j < n; j++) { double dx = ps[j].x - ps[i].x; double dy = ps[j].y - ps[i].y; double dd = dx * dx + dy * dy; int tmp1, tmp2; if (dd > 25.0) continue; else if (equal(dd, 25.0)) { Point center; center.x = (ps[i].x + ps[j].x) / 2.0; center.y = (ps[i].y + ps[j].y) / 2.0; tmp1 = count(center); } else { Line line = bisector(ps[i], ps[j]); double d = sqrt(6.25 - dd / 4.0); double em = sqrt(line.a * line.a + line.b * line.b); double ex = line.b / em; double ey = -line.a / em; double cx = (ps[i].x + ps[j].x) / 2.0; double cy = (ps[i].y + ps[j].y) / 2.0; Point center; center.x = cx + d * ex; center.y = cy + d * ey; tmp1 = count(center); center.x = cx - d * ex; center.y = cx - d * ey; tmp2 = count(center); tmp1 = tmp1 > tmp2 ? tmp1 : tmp2; } max = max < tmp1 ? tmp1 : max; } } printf("%dn", max); return 0;}
