import java.util.Scanner; public 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 total = 0; for(int i = 0 ; i < f ; i++) { a[i] = sc.nextInt(); total += a[i]; } if(k * n >= total) { System.out.println(k * n - total); } else { System.out.println(-1); } } }