import java.util.Scanner;

class no143{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		int k,n,f,a,i,s=0;
		k=stdIn.nextInt();
		n=stdIn.nextInt();
		f=stdIn.nextInt();
		for(i=0;i<f;i++) {
			a=stdIn.nextInt();
			s+=a;
		}
		f=k*n-s;
		if(f<0) i=-1;
		else i=f;
		System.out.print(i);
	}
}