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 sum = 0;
        int ans = 0;
        for(int i=0;i<F;i++)
        {
            int A = sc.nextInt();
            sum += A;
        }
        int all = K * N; 
        if(all<sum)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(all-sum);
        }
        
    }
}