#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <string>
#include <vector>
#include <cmath>
#include <set>
#include <map>
using namespace std;
using ll = long long;

int main(void)
{
  int K, N, F;
  cin >> K >> N >> F;
  int Sum = K * N;
  int Old = 0;
  vector<int> A(F);
  for (int i = 0; i < F; i++)
  {
    cin >> A.at(i);
    Old += A.at(i);
  }

  if (Old > Sum)
  {
    cout << -1 << endl;
  }
  else
  {
    cout << Sum - Old << endl;
  }

  return 0;
}