#include <bits/stdc++.h>
using namespace std;
int candles[1000];
int main() {
  int n, m, k;
  cin >> n >> m >> k;
  vector<int> ghosts(n);
  for (int i = 0; i < n; i++) {
    cin >> ghosts[i];
    ghosts[i] += 300;
  }
  int MaxCandles = 0;
  for (int i = 0; i < ghosts.size(); i++) {
    int Pos = ghosts[i];
    int start = Pos - 1;
    while (candles[Pos] < k) {
      if (candles[Pos] < k && start + m < Pos) {
        cout << "-1";
        return 0;
      }
      for (int j = start + 1; j <= start + m; j++) candles[j]++;
      start--;
      MaxCandles++;
    }
  }
  cout << MaxCandles;
  return 0;
}
