#UVa：10589－Area

計算點距離四頂點的距離是否都在a以內即可得解。

C++(0.100)

/*******************************************************/
/* UVa 10589 Area                                      */
/* Author: Maplewing [at] knightzone.studio            */
/* Version: 2011/12/17                                 */
/*******************************************************/
#include<iostream>
#include<cstdio>
using namespace std;

struct Point{
  double x;
  double y;
};

double distance2( Point p1, Point p2 ){
  return ((p1.x-p2.x)*(p1.x-p2.x))+((p1.y-p2.y)*(p1.y-p2.y));
}

int main(){
  int N, M, a;
  Point p, A, B, C, D;
  while( scanf( "%d%d", &N, &a ) != EOF && N ){
    M = 0;
    A.x = 0, A.y = 0;
    B.x = a, B.y = 0;
    C.x = a, C.y = a;
    D.x = 0, D.y = a;
    for( int i = 0 ; i < N ; i++ ){
      scanf( "%lf%lf", &p.x, &p.y );
      if( distance2( p, A ) <= (double)(a*a) &
          distance2( p, B ) <= (double)(a*a) &
          distance2( p, C ) <= (double)(a*a) &
          distance2( p, D ) <= (double)(a*a) )
        M++;
    }
    printf( "%.5lf\n", ((double)M/(double)N)*(double)a*(double)a );
  }
  return 0;
}