import java.util.*;


public class Main {
    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 sum = 0;
        
        for(int i=0; i<F; i++){
            sum += sc.nextInt();
        }
        sum = K*N - sum;
        if( sum >= 0){
            System.out.println( sum );
        }
        else{
            System.out.println( -1 );
        }
    }
}