import java.util.Scanner;
 
class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int k = sc.nextInt();
		int n = sc.nextInt();
		int f = sc.nextInt();
		int[] a = new int[f];
		int sum = 0;
		for(int i=0;i<f;i++){
			a[i] = sc.nextInt();
			sum += a[i];
		}
		int b = k * n;
		int ans;
		if(b<sum){
			ans = -1;
		} else {
			ans = b - sum;
		}
		System.out.println(ans);
	}
}