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();
    }
    int ans = Math.max(-1, K * N - sum);
    System.out.println(ans);
  }
}