import java.util.Scanner;

class Main{
    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 sig = K * N;
        
        int[] A = new int[F];
        
        for (int i = 0; i < F; i++){
            A[i] = scanner.nextInt();
        }
        
        for (int i = 0; i < F; i++){
            sig -= A[i];
        }
        
        if (sig < 0){
            System.out.println(-1);
        }
        
        else{
            System.out.println(sig);
        }
    }
}