#include <bits/stdc++.h>
using namespace std;
 
#define rep(i,n) for(int i=0;i<(n);++i)
#define loop for(;;)
#define trace(var) cerr<<">>> "<<#var<<" = "<<var<<endl;
#define inf (1e9)
#define eps (1e-9)
 
template<class S, class T> inline
ostream& operator<<(ostream&os, pair<S,T> p) {
  return os << '(' << p.first << ", " << p.second << ')';
}
 
template<class S, class T, class U> inline
ostream& operator<<(ostream&os, tuple<S,T,U> t) {
  return os << '('
    << get<0>(t) << ", "
    << get<1>(t) << ", "
    << get<2>(t) << ')';
}
 
template<class T> inline
ostream& operator<<(ostream&os, vector<T> v) {
  if (v.size() == 0) { return os << "(empty)"; }
  os << v[0];
  for (int i=1, len=v.size(); i<len; ++i) os << ' ' << v[i];
  return os;
}
 
template<class T> inline
istream& operator>>(istream&is, vector<T>&v) {
  rep (i, v.size()) is >> v[i];
  return is;
}
 
//           ^   >  v   <
int dx[] = { -1, 0, 1,  0 };
int dy[] = {  0, 1, 0, -1 };
 
typedef vector<int> vi;
int san = 0, ni = 0;
 
inline bool fullhouse(vi&xs) { return (san == 1) and (ni == 1); }
inline bool three(vi&xs) { return (san == 1) and (ni == 0); }
inline bool two(vi&xs) { return (san == 0) and (ni == 2); }
inline bool one(vi&xs) { return (san == 0) and (ni == 1); }
 
int main() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout.setf(ios::fixed);
  cout.precision(10);
 
  int k, n, f; cin >> k >> n >> f;
  int sum = 0;
  rep (i, f) { int x; cin >> x; sum += x; }
  int ans = k*n-sum;
  if (ans < 0) ans = -1;
  cout << ans << endl;
  return 0;

  int a,b,c,d,e;
  cin >> a >> b >> c >> d >> e;
  {
    int memo[20] = {0};
    vi xs = {a,b,c,d,e};
    for (int x: xs) memo[x]++;
    rep (i, 20) if (memo[i] == 2) ++ni; else if (memo[i] == 3) ++san;

    if (fullhouse(xs)) {
      cout << "FULL HOUSE" << endl;
    } else if (three(xs)) {
      cout << "THREE CARD" << endl;
    } else if (two(xs)) {
      cout << "TWO PAIR" << endl;
    } else if (one(xs)) {
      cout << "ONE PAIR" << endl;
    } else {
      cout << "NO HAND" << endl;
    }
  }
 
  return 0;
}