#include <stdio.h>

int main()
{
	enum Ryougae
	{
		ICHI = 25,
		NIJUGO = 4,
		HYAKU = 10,
	};
	int n = 0;
	int x = 0;
	int y = 0;
	int mame = 0;
	scanf("%d", &n);
	scanf("%d", &x);
	scanf("%d", &y);
	mame = n * x;

	for (int i = 0;i < y;i++)
	{
		int num = 0;
		scanf("%d", &num);
		mame -= num;
	}

	if (mame >= 0)
	{
		printf("%d", mame);
	}
	else
	{
		printf("%d", -1);
	}

	return 0;
}