#include <iostream>
#include <vector>

int main(int argc, char *argv[]) {
	int beans_per_pack = 0;
	int num_of_pack = 0;
	int num_of_members = 0;
	int member_age = 0;
	std::vector<int> member_ages;

	std::cin >> beans_per_pack;
	std::cin >> num_of_pack;
	std::cin >> num_of_members;

	while(std::cin >> member_age) {
		member_ages.push_back(member_age);
	}

	int all_beans = beans_per_pack  * num_of_pack;
	for (auto member_age: member_ages) {
		all_beans -= member_age;
	}

	std::cout << ((all_beans < 0) ? -1 : all_beans) << std::endl;
	return 0;
}