#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); ++i)

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