import java.io.*;
import java.text.*;
import java.util.*;
/**
 * Insert the type's description here.
 * Creation date: (11/11/00 12:44:28 AM)
 * @author: 
 */
public class Baz {

/**
 * Insert the method's description here.
 * Creation date: (11/11/00 12:47:28 AM)
 */
public Baz() {
	super();
	
}
/**
 * Starts the application.
 * @param args an array of command-line arguments
 */
public static void main(java.lang.String[] args) {

	String inputLine = null;
	BufferedReader reader = null;
	PrintWriter writer = null;
	StringTokenizer tokenizer = null;
	long numberOfCards = 0;
	long cardSum = 0;
	long sum = 0;
	int i = 0;

	try {
		reader = new BufferedReader(new InputStreamReader(System.in));
		//writer = new PrintWriter(System.out);

		//reader = new BufferedReader(new FileReader("c:\\temp\\baz.in"));
		writer = new PrintWriter(System.out);

		while (reader.ready()) {
			inputLine = reader.readLine();

			tokenizer = new StringTokenizer(inputLine);

			tokenizer.nextToken();
			numberOfCards = (new Long(tokenizer.nextToken())).longValue();
			sum = 0;
			cardSum = 0;
			for (i = 1; i < (numberOfCards); i++) {
				inputLine = reader.readLine();
				sum = sum + i;
				cardSum = cardSum + (new Long(inputLine)).longValue();
			}
			sum = sum + numberOfCards;
			
			writer.println("" + (sum - cardSum));
			writer.flush();
			reader.readLine();
		}
	} catch (Exception e) {
		//System.out.println("Exception occured: " + e.getMessage());
		//e.printStackTrace();
	}
}
}