package yukicoder;

import java.util.Scanner;

public class N143 {
	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 a;
		int sum = k * n;
		for(int i = 0;i < f;++i){
			a = sc.nextInt();
			sum -= a;
		}
		
		System.out.println(sum >= 0 ? sum : -1);
	}
}