import java.io.*;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;
 
public class Main {
    public static void main (String[] args) throws IOException
    {
    	Scanner sc = new Scanner(System.in);
    	
    	int K = sc.nextInt();
    	int N = sc.nextInt();
    	int F = sc.nextInt();
    	int S = 0;
    	
    	int af[] = new int[F+1];
    	
    	for (int i = 1 ; i <= F ; i++){
    		af[i] = sc.nextInt();
    	}
    	
    	for (int j = 1 ; j <= F ; j++){
    		S += af[j];
    	}
    	
    	if (K*N >= S){
    		System.out.println(K*N - S);
    	}
    	else {
    		System.out.println("-1");
    	}
    	
    	
    	
    }	
}