#include <iostream>
using namespace std;

int main(void)
{
	int K, N, F, sum = 0, tmp;
	cin >> K >> N >> F;
	for(int i = 0; i < F; i++) {
		cin >> tmp;
		sum += tmp;
	}
	int ans = K * N - sum;
	if(ans < 0) {
		cout << "-1" << endl;
	}
	else {
		cout << ans << endl;
	}
	return 0;
}