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