import java.util.Scanner;

class No_143{
	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];
		}
		
		if(sum > k * n){
		System.out.println(-1);
		}else{
		System.out.println(k * n - sum);
		}
		}
		}