#include <bits/stdc++.h>
using namespace std;
int m, t, r, x;
int tempo[650];
int ncandles[650], burn;
void acende(int j) {
  burn++;
  for (int i = (j + 1), _b = (j + t); i <= _b; i++) {
    ncandles[i]++;
  }
}
int main() {
  ios ::sync_with_stdio(false);
  cin >> m >> t >> r;
  bool flag = false;
  for (int i = 0, _a = (m); i < _a; i++) {
    cin >> x;
    tempo[x + 300] = 1;
  }
  if (t < r) {
    cout << -1 << endl;
  } else {
    int aux;
    for (int i = 0, _a = (650); i < _a; i++) {
      if (tempo[i] == 1) {
        aux = r - ncandles[i];
        for (int j = (i - aux), _b = (i - 1); j <= _b; j++) {
          acende(j);
        }
      }
    }
    cout << burn << endl;
  }
  return 0;
}
