#include <bits/stdc++.h>
using namespace std;

int main() {
  int k, n, f, a;
  int b;

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