import java.math.*;
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int k = sc.nextInt();
        int n = sc.nextInt();
        int f = sc.nextInt();
        int ans=k*n;
        for(int i=0;i<f;i++){
        	ans-=sc.nextInt();
        }
        System.out.println(Math.max(-1, ans));
    }
}