import java.util.Scanner; class No_143 { public static void main(String args[]) { Scanner stdIn = new Scanner(System.in); int beans = stdIn.nextInt(); int pack = stdIn.nextInt(); int family = stdIn.nextInt(); calicuLate(beans, pack, family); /*int sum = 0; for (int i = 0; i < family; i++) { sum += stdIn.nextInt(); } if ( beans * pack < sum ) { System.out.println(-1); } else { System.out.println((beans * pack) - sum); }*/ } public static int calicuLate(int beans, int pack, int family) { Scanner stdIn = new Scanner(System.in); int sum = 0; for (int i = 0; i < family; i++) { sum += stdIn.nextInt(); } if ( beans * pack < sum ) { System.out.println(-1); } else { System.out.println((beans * pack) - sum); } return 0; } }