#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

int main() {
  int K, N, F, x;
  int ans;
  cin >> K >> N >> F;
  ans = K * N;
  rep(i, 0, F) {
    cin >> x;
    ans -= x;
  }
  cout << (ans ? ans : -1) << endl;
  getchar();
}