import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.stream.Stream;

public class No00000143_Main {

	public static void main(String[] args) throws IOException {

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		int[] arr = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
		int zan = arr[0] * arr[1] - Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).sum();
		System.out.println(zan < 0 ? -1 : zan);
	}
}