import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.TreeSet;

public class Main {
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final int K = sc.nextInt();
		final int N = sc.nextInt();
		final int F = sc.nextInt();
		
		int rest = K * N;
		
		for(int i = 0; i < F; i++){
			final int A = sc.nextInt();
		
			rest -= A;
		}
		
		rest = Math.max(-1, rest);
		
		System.out.println(rest);
		
	}
}