source_code: import java.io.*;
  import java.util.*;
  
  public class Main {
  
      StreamTokenizer in;
      PrintWriter out;
  
      public static void main(String[] args) throws Exception {
          new Main().run();
      }
  
      public void run() throws Exception {
          in = new StreamTokenizer (new BufferedReader(new InputStreamReader(System.in)));
          out = new PrintWriter(new OutputStreamWriter(System.out));
          solve();
          out.flush();
      }
      
      int nextInt() throws Exception {
          in.nextToken();
          return (int) in.nval;
      }
  
  
      public void solve() throws Exception {
          int n=nextInt();
          long ans=0;
          for (int i=0;i<n;i+=2)
              ans+=3;
          out.println(ans);
      }
  }