Submission #204465

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Source Code Expand

#include <iostream>
#include <vector>
#include <cstdio>
using namespace std;

int main()
{
  vector<vector<double> > pascal(1001, vector<double>(0));
  pascal[0].push_back(1.0);
  double side = 1.0;
  for (unsigned int i = 1; i < pascal.size(); ++i) {
    side /= 2.0;
    pascal[i].push_back(side);
    for (unsigned int j = 1; j < pascal[i-1].size(); ++j) {
      pascal[i].push_back((pascal[i-1][j-1] + pascal[i-1][j]) / 2.0); 
    }
    pascal[i].push_back(side);
  }

  int N, D, X, Y;
  while (cin >> N >> D >> X >> Y) {
    if (X < 0)
      X = -X;
    if (Y < 0)
      Y = -Y;

    if (X % D != 0 || Y % D != 0) {
      cout << "0" << endl;
      continue;
    }
    X /= D;
    Y /= D;

    double ans = 0.0;
    for (int lr = 0; lr <= N; ++lr) {
      int ud = N - lr;

      if (lr < X || ud < Y || (lr - X) % 2 != 0 || (ud - Y) % 2 != 0)
        continue;

      ans += pascal[N][lr] * pascal[lr][(lr - X) / 2] * pascal[ud][(ud - Y) / 2];
    }
    printf("%.10f\n", ans);
  }
  return 0;
}
 

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