import java.io.*;
import java.util.*;

class Main143 {
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
			
		String[] line1 = br.readLine().split(" ");
		int k = Integer.parseInt(line1[0]);
		int n = Integer.parseInt(line1[1]);
		int f = Integer.parseInt(line1[2]);
		int beans = k * n, sum = 0;
		String[] line2 = br.readLine().split(" ");
		for (String s : line2)
			sum += Integer.parseInt(s);
		
		System.out.println(sum <= beans ? beans - sum : -1);
	}
}