#include <bits/stdc++.h>
using namespace std;
long long MOD = 1e9 + 7;
long long MAX = 1e17;
bool b[1000];
int main() {
  ios_base::sync_with_stdio(0);
  ;
  int m, t, r, ans = 0;
  cin >> m >> t >> r;
  int w[m];
  for (int i = 0; i < m; i++) cin >> w[i];
  if (r > t) {
    cout << -1 << endl;
    return 0;
  }
  for (int i = 0; i < m; i++) w[i] += t + 10;
  for (int i = 0; i < m; i++) {
    int k = r;
    for (int j = w[i] - t; j < w[i]; j++)
      if (b[j]) k--;
    if (k <= 0) continue;
    for (int j = w[i] - 1; j > w[i] - t - 1; j--) {
      if (!b[j]) {
        b[j] = 1;
        k--;
        ans++;
      }
      if (k == 0) break;
      if (j == w[i] - t) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  cout << ans << endl;
}
