package yukiCoder;

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

public class No00143 {

	public static void main(String[] args) {
		BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

		String[] input1 = new String[3];


		try {
			input1 = bufferedReader.readLine().split(" ");
			int[] input2 = new int[Integer.parseInt(input1[2])];
			input2 = toIntArray(bufferedReader.readLine().split(" "));
			int fullBeans = Integer.parseInt(input1[0]) * Integer.parseInt(input1[1]);
			System.out.println(eatFamily(fullBeans, input2));

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

	public static int[] toIntArray(String[] stringArray) {
		int[] intArray = new int[stringArray.length];
		for (int i = 0 ; i < stringArray.length ; i++) {
			intArray[i] = Integer.parseInt(stringArray[i]);
		}
		return intArray;
	}

	public static int eatFamily(int fullBeans, int[] ageArray) {
		for (int i = 0 ; i < ageArray.length ; i++) {
			fullBeans -= ageArray[i];
			if (Integer.signum(fullBeans) == -1) {
				return -1;
			}
		}
		return fullBeans;
	}

}