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

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

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		
		int mame,hukuro,human,old=0;
		mame = sc.nextInt();
		hukuro = sc.nextInt();
		mame *= hukuro;
		human = sc.nextInt();
		
		for(int i=0;i<human;i++){
			old += sc.nextInt();
		}
		
		mame -= old;
		if(mame>=0){
			System.out.println(mame);
		}else{
			System.out.println(-1);
		}
		
	}
}