import java.io.*;
import java.util.*;
import java.util.stream.*;

// 処理
class Process {
    private int K;
    private int N;
    private int[] A;

    Process(int K, int N, int[] A) {
        this.K = K;
        this.N = N;
        this.A = A;
    }

    int getResult() {
        int beansCount = (K * N);

        int ageTotal = IntStream.of(A).sum();

        return ((beansCount >= ageTotal) ? (beansCount - ageTotal) : -1);
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        var bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        var printWriter    = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        // 入力
        int[] input = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();

        int[] A = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();

        // 出力
        printWriter.println((new Process(input[0], input[1], A)).getResult());

        bufferedReader.close();
        printWriter.close();
    }
}