#include<iostream>
#include<math.h>
#include<string>
using namespace std;

int main(){
	int k;
	int n;
	int f;
	int a;
	int total = 0;
	cin >> k;
	cin >> n;
	cin >> f;
	for (int i = 0; i < f; i++){
		cin >> a;
		total += a;
	}
	if (k*n >= total){
		cout << k*n - total << endl;
	}
	else{
		cout << "-1" << endl;
	}
	return 0;
}