source_code: import java.util.*;
  import java.io.*;
  
  public class Soldiers {
  	public static void main(String[] args) throws IOException {
  		new Soldiers().run();
  	}
  
  	void run() throws IOException {
  		br = new BufferedReader(new InputStreamReader(System.in));
  		pw = new PrintWriter(System.out);
  
  		int n = nextInt();
  		pw.println(3 * (n / 2));
  
  		pw.close();
  	}
  
  	BufferedReader br;
  	StringTokenizer st;
  	PrintWriter pw;
  
  	String next() throws IOException {
  		while (st == null || !st.hasMoreTokens()) {
  			st = new StringTokenizer(br.readLine());
  		}
  		return st.nextToken();
  	}
  
  	int nextInt() throws IOException {
  		return Integer.parseInt(next());
  	}
  
  	double nextDouble() throws IOException {
  		return Double.parseDouble(next());
  	}
  }
  