import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int K = sc.nextInt();
        int N = sc.nextInt();
int sum = K*N;
        int F = sc.nextInt();
        int [] A = new int [F];
int total =0;
for(int i=0; i<F; i++){
A[i]=sc.nextInt();
total = total + A[i];
}
int r = sum-total;
if(r<0){
r=-1;
}
        System.out.println(r);
    }
}