source_code: import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  
  /**
   * May 13, 2011 
   * @author parisel
   */
  public class ToyArmy {
      int N;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      String[] tok;
      String s;
  
      private String[] getTok() throws IOException {return br.readLine().split(" ");}
      private int getInt() throws IOException {return Integer.valueOf(br.readLine());}
      private int[] getInt(int N) throws IOException {
          int[] data= new int[N]; tok= br.readLine().split(" ");
          for (int i=0; i<N; i++) data[i]= Integer.valueOf(tok[i]);
          return data;
      }
  
      public void solve() throws IOException {
          int i=0, j=0;
          N= getInt();
  
          long kill= (3*N)/2;
  
          System.out.printf("%d\n", kill);
  
      }
  
  
      public static void main(String[] args) throws IOException {
          new ToyArmy().solve();
      }
  }
  