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  1. #include <algorithm>
  2. #include <cstdio>
  3. #include <stdint.h>
  4.  
  5. using namespace std;
  6.  
  7. struct point
  8. {
  9.     int x, y;
  10.  
  11. 	bool top (void) const
  12. 	{
  13. 		return y > 0 || (y == 0 && x > 0);
  14. 	}
  15.  
  16. 	bool operator < (point const & other) const
  17. 	{
  18. 		if (top () != other.top ())
  19. 		{
  20. 			return top () > other.top ();
  21. 		}
  22. 		return (int64_t) x * other.y < (int64_t) y * other.x;
  23. 	}
  24. };
  25.  
  26. int main (void)
  27. {
  28. 	int n;
  29. 	while (scanf (" %d", &n) != EOF)
  30. 	{
  31. 		int m = n - 1;
  32. 		point b [n];
  33. 		for (int i = 0; i < n; i++)
  34. 		{
  35. 			scanf (" %d %d", &b[i].x, &b[i].y);
  36. 		}
  37.  
  38. 		double res = 0.0;
  39. 		for (int i = 0; i < n; i++)
  40. 		{
  41. 			point a [m * 2];
  42. 			for (int j = 0; j < n; j++)
  43. 			{
  44. 				if (j != i)
  45. 				{
  46. 					a[j - (j > i)].x = b[j].x - b[i].x;
  47. 					a[j - (j > i)].y = b[j].y - b[i].y;
  48. 				}
  49. 			}
  50. 			sort (a, a + m);
  51. 			for (int j = 0; j < m; j++)
  52. 			{
  53. 				a[j + m].x = a[j].x;
  54. 				a[j + m].y = a[j].y;
  55. 			}
  56.  
  57. 			double sumx = a[0].x;
  58. 			double sumy = a[0].y;
  59. 			int k = 1;
  60. 			for (int j = 0; j < m; j++)
  61. 			{
  62. 				sumx -= a[j].x;
  63. 				sumy -= a[j].y;
  64. 				while (((int64_t) a[j].x * a[k].y <
  65. 				        (int64_t) a[j].y * a[k].x) ||
  66. 				       j == k)
  67. 				{
  68. 					sumx += a[k].x;
  69. 					sumy += a[k].y;
  70. 					k++;
  71. 				}
  72. 				res += a[j].y * sumx - a[j].x * sumy;
  73. 			}
  74. 		}
  75.  
  76. 		res /= n;
  77. 		res /= (n - 1);
  78. 		res /= (n - 2);
  79. 		printf ("%.20lf\n", res);
  80. 	}
  81. 	return 0;
  82. }
  83.  
Success #stdin #stdout 0.47s 2860KB
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stdin
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2000
stdout
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6673906517063116.00000000000000000000
https://ideone.com/Pf5v4i
language:
C++ (gcc 8.3)
created:
10 years ago
visibility:
secret

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