import java.util.Scanner;

class Test {
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		int k = in.nextInt();
		int n = in.nextInt();
		int f = in.nextInt();

		int[] a = new int[f-1];
		int temp = 0;
		for(int i=0; i<f; i++) {
			a[i] = in.nextInt();
			temp += a[i];
		}

		if(k * n < temp) {
			System.out.println(-1);
		} else {
			System.out.println(k * n - temp);
		}
	}
}