public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader in  = new BufferedReader(new InputStreamReader(System.in));
		int length = Integer.parseInt(in.readLine());
		String[] strArray = in.readLine().split(" ");
		int[] array = new int[length];
		for (int i = 0; i < strArray.length; i++) {
			array[i] = Integer.parseInt(strArray[i]);
		}
		
		missingNumber(array);

	}