#include<iostream>
using namespace std;
int main() {
	int K, N, F;
	int A[101];
	cin >> K >> N >> F;
	for (int i = 1; i <= F; i++) {
		cin >> A[i];
	}
	int nuts = K * N;
	for (int i = 1; i <= F; i++) {
		nuts -= A[i];
		if (nuts < 0) {
			nuts = -1;
			break;
		}
	}
	cout << nuts << endl;
	return 0;
}