import java.util.Scanner;

class No143{
     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 x[] = new int[f];
         int eatSoy = 0;
         int pickSoy = k*n;

         for(int i=0; i<f; i++){
             x[i] = sc.nextInt();
             eatSoy += x[i];
         }

         if(pickSoy<eatSoy){
             System.out.print("-1");
         }
         else{
             System.out.print(pickSoy-eatSoy);
         }
     }
}