import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

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

		proconScan proScan = new proconScan(" ");
		proScan.scan();
		Integer numK = proScan.nextInt();
		Integer numN = proScan.nextInt();
		//Integer numF = proScan.nextInt();
		proScan.scan();
		Integer sum = 0;
		for (Integer buf : proScan.getInt()) {
			sum += buf;
		}
		Integer buf = numK * numN;
		if(buf>=sum){
			System.out.print(buf-sum);
		}else{
			System.out.print("-1");
		}
		proScan.proconEnd();
	}

}


class proconScan {
	private BufferedReader br;
	private String inLine;
	private String[] inString;
	private Integer idx;
	private String splitChar;

	public proconScan() {
		br = new BufferedReader(new InputStreamReader(System.in));
		idx = 0;
		splitChar = " ";
	}

	public proconScan(String split) {
		br = new BufferedReader(new InputStreamReader(System.in));
		idx = 0;
		splitChar = split;
	}

	public void proconEnd() throws IOException {
		// TODO 自動生成されたメソッド・スタブ
		br.close();
	}

	public String scan() throws IOException {
		idx = 0;
		inLine = br.readLine();
		inString = inLine.split(splitChar, 0);
		return inLine;
	}

	public Integer scanInteger1() throws IOException {
		scan();
		return Integer.parseInt(inString[0]);
	}

	public String backLine() {
		return inLine;
	}

	public String nextString() {
		return inString[idx++];
	}

	public String[] getString() {
		return inString;
	}

	public Integer nextInt() {
		Integer num;
		try {
			num = Integer.parseInt(inString[idx++]);
		} catch (NumberFormatException e) {
			num = null;// TODO: handle exception
		}
		return num;
	}

	public Integer[] getInt() {
		Integer[] num = new Integer[this.getLen()];

		for (int i = 0; i < this.getLen(); i++) {
			num[i] = Integer.parseInt(inString[i]);
		}

		return num;
	}

	public Float nextFloat() {
		Float num;
		try {
			num = Float.parseFloat(inString[idx++]);
		} catch (NumberFormatException e) {
			num = null;// TODO: handle exception
		}
		return num;
	}

	public Float[] getFloat() {
		Float[] num = new Float[this.getLen()];

		for (int i = 0; i < this.getLen(); i++) {
			num[i] = Float.parseFloat(inString[i]);
		}

		return num;
	}

	public Integer getLen() {
		return inString.length;
	}

}