import java.util.Scanner;

class Wandbox
{
    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 ans = k * n;
    	
    	for (int i = 0; i < f; i++) {
    		ans -= sc.nextInt();
    	} 
    	if (ans >= 0) {
    		System.out.println(ans);
    	} else {
    		System.out.println(-1);
    	}
    	
    }
}