import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        // 標準入力から読み込む際に、Scannerオブジェクトを使う。
        Scanner sc = new Scanner(System.in);

        int k = sc.nextInt();

        int n = sc.nextInt();
        
        int f = sc.nextInt();
        
        int total = k * n;
        
        int eat = 0;
        
        int remaining = 0;
        
        for(int i = 0; i < f; i++) {
            int a = sc.nextInt();
            total -= a;
        	
        }
        
        if(total  <= 0) {
            System.out.println(-1);
            
        }else {
            System.out.println(total);
        }
    }
}