/*************************************************************************
 *  Compilation:  javac RandomWalkers.java
 *  Execution:    java RandomWalker N
 *  
 *  Simulates how long it takes N random walkers starting at the center
 *  of an N-by-N grid to visit every cell in the grid.
 *
 *************************************************************************/

public class RandomWalkers { 

   public static void main(String[] args) {
      int N = Integer.parseInt(args[0]);
      int[] x = new int[N];         // x positions
      int[] y = new int[N];         // y positions
      int cellsToVisit = N*N;       // cells left to visit
      int steps = 0;                // number of steps taken
      double r;
      boolean[][] visited = new boolean[N][N];  // has the i-j been visited?

      // start at center
      for (int i = 0; i < N; i++)
         x[i] = y[i] = N/2;
      visited[N/2][N/2] = true;
      cellsToVisit--;


      // repeat until all cells have been visited
      while(cellsToVisit > 0) {
         steps++;

         // move random walker i
         for (int i = 0; i < N; i++) {
            r = Math.random();
            if      (r <= 0.25) x[i]++;
            else if (r <= 0.50) x[i]--;
            else if (r <= 0.75) y[i]++;
            else if (r <= 1.00) y[i]--;

            // check if (x[i], y[i]) is inside N-by-N boundary and has been visited
            if (x[i] < N && y[i] < N && x[i] >= 0 && y[i] >= 0 && !visited[x[i]][y[i]]) {
               cellsToVisit--;
               visited[x[i]][y[i]] = true;
            }
         }
      }

      System.out.println(steps);
   }

}