package P361;

import java.util.Scanner;

public class P361 {
	public static void main(String args[]){
		
		Scanner sc = new Scanner(System.in);
		String r = sc.nextLine();
		String[] rstrs = r.split(" ",0);
		String s = sc.nextLine();
		String[] sstrs = s.split(" ",0);
		
		int k = Integer.parseInt(rstrs[0]);
		int n = Integer.parseInt(rstrs[1]);
		int f = Integer.parseInt(rstrs[2]);
		
		int[] a = new int[f];
		int sumk = k * n;
		int needk = 0;
		
		for (int i=0;i<f;i++) {
			a[i] = Integer.parseInt(sstrs[i]);
			needk += a[i];
		}
		
		int ans = 0;
		
		if (sumk < needk) {
			ans = -1;
		} else if (sumk >= needk) {
			ans = sumk - needk;
		}
		
				System.out.println(ans);
	}
	
}