#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>

using namespace std;
int main() {
	int K,N,F,i;
	cin >> K >> N >> F;
	vector<int> A(F);
	for (i = 0; i < F; i++) { cin >> A[i]; }
	if (K*N >= accumulate(A.begin(), A.end(), 0)) cout << K*N - accumulate(A.begin(), A.end(), 0);
	else cout << -1;
	return 0;
}