source_code: import java.io.BufferedReader;
  import java.io.InputStreamReader;
  public class Main {
  	public static void main(String args[]) throws Exception
  	{
  		BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
  		int num=Integer.parseInt(in.readLine());
  		System.out.println((num/2)*(3));
  	}		
  
  }
  