#include <iostream>
using namespace std;

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