/*No.143 豆*/
import java.util.*;
public class No143 {
	public static void main(String[] args) {
		byte family,age_eat=0;
		short tsubu,fukuro;
		int mame;
		try(Scanner input = new Scanner(System.in)){
			tsubu=input.nextShort();
			fukuro=input.nextShort();
			family=input.nextByte();
			byte age[]=new byte[family];
			for(byte i=0;i<age.length;i++){
				age[i]=input.nextByte();
			}
			Arrays.sort(age);
			mame=tsubu*fukuro;
			for(byte j=0;j<age.length;j++){
				if(mame<age[j]){
					mame=0;
					break;
				}else if(mame==age[j]){
					mame=0;
					age_eat++;
					break;
				}else if(mame>age[j]){
					mame-=age[j];
					age_eat++;
				}					
			}
			if(age_eat==0){
				System.out.println(-1);
			}else{
				System.out.println(mame);
			}
		}catch(Exception e){
			e.printStackTrace();
		}
	}
}