import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int K = sc.nextInt(); //粒
        int N = sc.nextInt(); //袋
        int F = sc.nextInt(); //家族
        
        int []A = new int[F];
        for(int i =0; i<F; i++){
            A[i] = sc.nextInt();
        }
        
        //合計の粒
        int all_beans = K*N;
        
        int all_old = 0;
        for(int i=0; i<F; i++){
            all_old += A[i];
        }
        
        if(all_beans - all_old>=0){
            System.out.println(all_beans - all_old);
        }else{
            System.out.println(-1);
        }
        
    }
}