import java.util.Scanner;

public class No143 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lineSplit = line.split(" ");
		int k = Integer.parseInt(lineSplit[0]);
		int n = Integer.parseInt(lineSplit[1]);
		int f = Integer.parseInt(lineSplit[2]);

		String line2 = sc.nextLine();
		String[] family = line2.split(" ");
		int beans = k * n;

		for(int i = 0; i < family.length; i++){
			int age = Integer.parseInt(family[i]);
			beans -= age;
			if(beans < 0){
				beans = -1;
			}
		}
		System.out.println(beans);
	}

}