import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) {
	      BufferedReader stdReader =new BufferedReader(new InputStreamReader(System.in));
	      
	      try {
	    	  String[] temps = stdReader.readLine().split(" ");
	    	  int k=Integer.parseInt(temps[0]);
	    	  int n=Integer.parseInt(temps[1]);
	    	  int f=Integer.parseInt(temps[2]);
	    	  int sumBeans = k*n;
	    	  String[] temps2 = stdReader.readLine().split(" ");
	    	  int sumAge = 0;
	    	  for(int i=0;i<temps2.length;i++){
	    		  sumAge+=Integer.parseInt(temps2[i]);
	    	  }
	    	  if(sumBeans-sumAge>=0){
	    		  System.out.println(sumBeans-sumAge);
	    	  }else{
	    		  System.out.println(-1);
	    	  }
	      } catch (IOException e) {
			e.printStackTrace();
	      }
	}
}