import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.stream.Stream;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        String[] inputs = reader.readLine().split(" ");
        int K = Integer.parseInt(inputs[0]);
        int N = Integer.parseInt(inputs[1]);
        int F = Integer.parseInt(inputs[2]);

        int sum = K * N;
        inputs =  reader.readLine().split(" ");
        int[] eatBeans = Stream.of(inputs).mapToInt(Integer::parseInt).toArray();
        for (int i = 0; i < F; i++) {
            sum -= eatBeans[i];
        }
        if(sum <0) {
            System.out.println(-1);
        } else {
            System.out.println(sum);
        }

    }
}