#include <iostream>		//cin, cout
#include <vector>		//vector
#include <algorithm>	//sort,min,max,count
#include <string>		//string,getline, to_string

using namespace std;

int main() {

	int K, N, F;
	cin >> K >> N >> F;

	int ans = K * N;
	
	int temp;
	for (int i = 0; i < F; i++) {
		cin >> temp;
		ans = ans - temp;
	}

	if (ans >= 0) {
		cout << ans << endl;
	}
	else {
		cout << -1 << endl;
	}

	return 0;

}