import java.util.Scanner;

public class Main {
	public static void main(String[] a) {
		Scanner s = new Scanner(System.in);
		int i=0,K=s.nextInt(),S=K*s.nextInt(),F=s.nextInt();
		for(;F>i++;)S-=s.nextInt();
		System.out.println(S<0?-1:S);
	}
}