#include<iostream>
using namespace std;
int main()
{ double P =1.0;
  cout.precision(10);
  for (double x = 2; x <= 20000; x=x+2)
  {//cout << "x=" <<x <<endl;
   P=P*x*x/(x-1)/(x+1);
   //cout << "P=" << P << endl;
  }
  cout << " wallis PI = " << P*2 << endl;
  cin.get();
  return 0;
}