import java.util.ArrayList;
import java.util.Scanner;

public class Beanse {
	static Scanner scan= new Scanner(System.in);
	public static void main(String[] args) {
		int K = scan.nextInt();
		int N = scan.nextInt();
		int F = scan.nextInt()-1;
		ArrayList<Integer> A = new ArrayList<Integer>();
		for(int i=0;i<=F;i++){
			A.add(scan.nextInt());
		}
		
		int TotalBeans = K*N;
		for(int i=0;i<=F;i++){
			TotalBeans -= (int)A.get(i);
		}
		if(TotalBeans<0){
			System.out.println(-1);
		}else{
			System.out.println(TotalBeans);
		}
	}
}