#include <iostream>

using namespace std;

int main(){

	int K,N,F;
	int age = 0;
	int num = 0;

	cin >> K >> N >> F;

	num = K*N;

	for(int i = 0; i < F; i++){
		cin >> age;
		num += -age;
	}
	cout << (num >= 0 ? num : -1) << endl;

	return 0;
}