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

public class Main {
	public static void main(String[] args) {
		BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
		try {

			String[] line = read.readLine().split(" ");
			int count = Integer.parseInt(line[0]) * Integer.parseInt(line[1]);
			int f = Integer.parseInt(line[2]);

			line = read.readLine().split(" ");
			int total = 0;
			for(int i = 0; i < f; ++i){
				total += Integer.parseInt(line[i]);
			}

			if(total > count){
				System.out.println(-1);
			}else{
				System.out.println(count - total);
			}

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}