import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner scanner = new Scanner(System.in);
		int K = scanner.nextInt();
		int N = scanner.nextInt();
		int F = scanner.nextInt();
		int beansSum = K * N;
		int ageSum = 0;
		
		for(int i = 0;i < F;i++){
			ageSum += scanner.nextInt();
		}
		if(beansSum < ageSum ){
			System.out.println(-1);
		}else{
			System.out.println(beansSum - ageSum);
		}
		scanner.close();
	}
}