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

class Main
{
	public static void main (String[] args) 
	{
		// 入力値の読み込み
		Scanner sc = new Scanner(System.in);
		int  k = sc.nextInt();
		int  n = sc.nextInt();
		int  f = sc.nextInt();
		
		int nt=0;
		
		int[]  a = new int[f];
		for(int m=0;m < f;m++){
		a[m] = sc.nextInt();
		nt += a[m];
		}
		
		int total = k*n;
		
		if(nt > total){
		System.out.println("-1");
		}else {
		System.out.println(total-nt);
		}
	}
}