import java.util.Scanner;

public class Beans {

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