#include <bits/stdc++.h>
using namespace std;
const int val = 2e5 + 7, val2 = 1e9 + 7;
int main() {
  int m, t, r, X[300], tim[601] = {0}, ans;
  cin >> m >> t >> r;
  for (int i = 0; i < m; i++) {
    cin >> X[i];
    X[i] += 300;
  }
  if (t < r)
    cout << -1;
  else {
    ans = 0;
    for (int i = 0; i < m; i++) {
      int cnt = 0;
      for (int j = X[i] - 1; j >= X[i] - t; j--) cnt += tim[j];
      if (cnt < r)
        for (int j = X[i] - 1; j >= X[i] - t && cnt < r; j--)
          if (tim[j] == 0) tim[j] = 1, cnt++, ans++;
      if (cnt < r) {
        cout << -1;
        break;
      }
    }
    cout << ans;
  }
  return 0;
}
