import java.util.*;

public class paiza7 {
    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 age[] = new int[F];
		int TotalAge = 0;
		
		for(int i=0;i<F;i++){
			age[i] = sc.nextInt();
			TotalAge += age[i];
		}
		
		if((K*N)>=TotalAge){
			System.out.println(K*N-TotalAge);
		}else{
			System.out.println("-1");
		}
    }
}