import java.util.Scanner;

public class No143 {
	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 beans = k * n;
		for(int j = 0; j < age.length ; j++){
			age[j] = sc.nextInt();
			beans -= age[j];
		}
		System.out.println( (beans >= 0) ? beans : -1 );
	}
}