import java.util.*;
public class No143 {
    public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		int k = sc.nextInt();
		int n = sc.nextInt();
		int f = sc.nextInt();
		k = k * n;
		for (int i = 0; i < f; i++){
			k -= sc.nextInt();
		}
		System.out.println(k < 0?-1:k);
		}
}