code: import static java.lang.Math.pow;
  import java.util.*;
  import java.util.function.Function;
  
  public class Test {
      static double calc(Function<Integer, Integer[]> f, int n) {
          double temp = 0;
  
          for (int ni = n; ni >= 1; ni--) {
              Integer[] p = f.apply(ni);
              temp = p[1] / (double) (p[0] + temp);
          }
          return f.apply(0)[0] + temp;
      }
  
      public static void main(String[] args) {
          List<Function<Integer, Integer[]>> fList = new ArrayList<>();
          fList.add(n -> new Integer[]{n > 0 ? 2 : 1, 1});
          fList.add(n -> new Integer[]{n > 0 ? n : 2, n > 1 ? (n - 1) : 1});
          fList.add(n -> new Integer[]{n > 0 ? 6 : 3, (int) pow(2 * n - 1, 2)});
  
          for (Function<Integer, Integer[]> f : fList)
              System.out.println(calc(f, 200));
      }
  }
  