import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int K = sc.nextInt(), N = sc.nextInt(), F = sc.nextInt();
		
		int[] A = new int[F];
		
		for(int i = 0; i < F; i++) {
			A[i] = sc.nextInt();
		}
		
		int sum = K * N;
		
		for(int i = 0; i < F; i++) {
			sum-=A[i];
		}
		
		if(sum < 0) {
			System.out.println(-1);
		} else {
			System.out.println(sum);
		}
		
	}
}