import java.util.Scanner;

public class No143 {
    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[] A = new int[F];
        int sum = 0, ans;
        for (int i = 0; i < F; i++) {
            A[i] = sc.nextInt();
            sum += A[i];
        }

        ans = K * N - sum;
        if(ans < 0){
            ans = -1;
        }
        System.out.println(ans);
    }

    
}