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

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		//K N F
		int[] teisu = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
		int count = teisu[0] * teisu[1];
		//年齢
		int sum = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).sum();

		System.out.println((count - sum) > 0 ? count - sum : "-1");
	}
}