import java.util.Scanner;

public class No143 {
	public static void main(String[]args) {
		Scanner sc = new Scanner(System.in);

		int K,N,F;
		K = sc.nextInt();
		N = sc.nextInt();
		F = sc.nextInt();

		int []a = new int[F];
		int B=0 ;

		for(int i=0;i<F;i++) {
			a[i] = sc.nextInt();
			B +=a[i];
		}

		int S ;
		S = K * N;

		if(S-B>=0) {
			System.out.println(S-B);
		}
		else {
			System.out.println(-1);
		}


	}

}