import java.util.*;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int k = sc.nextInt();
		int n = sc.nextInt();
		int mame = k*n;
		int f = sc.nextInt();
		int sum = 0;
		for(int i=0;i<f;i++){
			sum += sc.nextInt();
		}
		if(mame>=sum){
			System.out.println(mame-sum);
		}else{
			System.out.println(-1);
		}
	}
}