/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc = new Scanner(System.in);
    	int K = sc.nextInt();
    	int N = sc.nextInt();
    	// 人数
    	int F = sc.nextInt();
    	
    	int sum = 0;
    	
    	for (int i=0;i < F;i++) {
    		 sum += sc.nextInt();	
    	}
    	System.out.println((0 <= K * N - sum) ? K * N - sum : -1);
	}
}