source_code: import java.io.*;
  import java.util.*;
  import java.math.BigInteger;
  
  public class Main implements Runnable {
    private final String problemname = "";
    private final String FILE_IN  = problemname + ".in";
    private final String FILE_OUT = problemname + ".out";
  
    public void run() {
  //    in = new BufferedReader(new FileReader(FILE_IN));
  //    out = new PrintWriter(FILE_OUT);
      in = new BufferedReader(new InputStreamReader(System.in));
      out = new PrintWriter(System.out);
      _st = new StringTokenizer("");
  
      // TODO: type your code here
      int n = nextInt();
  
      int best = n + n / 2;
      /*
      for (int i = 0; i <= n; i++) {
        for (int j = 0; j <= n - i; j++) {
          best = Math.max(best, i + j + Math.min(n - i, n - j));
        }
      }*/
  
      out.println(best);
  
      try {
        in.close();
        out.close();
      }
      catch (Exception e) {
        System.err.println("Epic fail");
      }
    }
  
    BufferedReader in;
    PrintWriter out;
    StringTokenizer _st;
  
    public static void main(String[] args) {
      new Thread(new Main()).start();
    }
  
    private void seek() {
      while (!_st.hasMoreTokens()) {
        String s;
        try {
          s = in.readLine();
          _st = new StringTokenizer(s);
        } catch (Exception e) {
          return;
        }
      }
    }
  
    private String next() {
      seek();
      return _st.nextToken();
    }
  
    private int nextInt() {
      return Integer.parseInt(next());
    }
  
    private long nextLong() {
      return Long.parseLong(next());
    }
  
    private double nextDouble() {
      return Double.parseDouble(next());
    }
  
    private BigInteger nextBigInteger() {
      return new BigInteger(next());
    }
  }
  