import java.util.Scanner; public class Main{ public static void main(String[] args) { //System.out.println("袋に入っている豆の数"); Scanner scanner = new Scanner(System.in); int K = scanner.nextInt(); //System.out.println("袋の数"); int N = scanner.nextInt(); if(N==3) { System.out.println("daada"); } //System.out.println("家族の人数"); int F = scanner.nextInt(); int b = K*N; //豆の数 int []A = new int[F]; //家族の年齢 int e = 0; //必要な豆の数 for(int i = 0; i < F; i++) { A[i] = scanner.nextInt(); e += A[i]; } scanner.close(); if(e <= b) { System.out.println(b - e); }else { System.out.println(-1); } } }