import java.util.*;

public class Main {
	public static void main(String[] args) {

		int o,p,q;
		int a = 0, b = 0;
		
		Scanner sc  = new Scanner(System.in);
//		int[] test  = new int[100000];
  		int[] test2 = new int[1000];
//	  String str = sc.next();
	    
	  int	s = Integer.parseInt(sc.next());
		int	t = Integer.parseInt(sc.next());
	  int	u = Integer.parseInt(sc.next());
		
	/*	
		for(int i = 0; i < str.length() ; i++ ){ 
		
	    o = Integer.valueOf(s).intValue();
				b += o;
        				
				
				
		}

		System.out.println(b);
		
   */
    int d = 0;
		while(true){
		   if(d==u)break;
       int	v = Integer.parseInt(sc.next());
       test2[d] = v ;
       d++;
		}
		
		for(int i = 0; i < u; i++){
			a += test2[i];
		}
		if((s*t)-a >= 0)	 System.out.println((s*t)-a);
		else  	 System.out.println("-1");
		System.out.println();
	}
}