import java.util.Scanner;

public class Practice {
    public static void main(String args[]){
        Scanner scanner=new Scanner(System.in);
        int K=scanner.nextInt(),N=scanner.nextInt(),F=scanner.nextInt();
        int Fsum=0;
        for(int i=0;i<F;i++){
            Fsum+=scanner.nextInt();
        }
        if(K*N-Fsum<0){
            System.out.println(-1);
        }else{
            System.out.println(K*N-Fsum);
        }
    }
}