#include <iostream>
using namespace std;

int main() {
  int k, n, f, a;
  cin >> k >> n >> f;
  int beans = k * n;
  int total_age = 0;
  for (size_t i = 0; i < f; i++) {
    cin >> a;
    total_age += a;
  }
  int ans = beans - total_age;
  if (ans >= 0) {
    cout << ans << endl;
  } else {
    cout << -1 << endl;
  }
}