#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
long double PI = 4 * atan(1);
int m, t, r, w[301], g[1001], c[1001], cnt[1001], rq, ans;
int main() {
  ios::sync_with_stdio(0), cin.tie(0);
  cin >> m >> t >> r;
  for (int i = 0; i < m; i++) cin >> w[i];
  sort(w, w + m);
  for (int i = 0; i < m; i++) {
    g[300 + w[i]] = 1;
  }
  for (int i = 301; i < 601; i++) {
    if (g[i]) {
      rq = max(0, r - cnt[i]);
      while (rq--) {
        for (int j = i - 1; j >= 1; j--) {
          if (!c[j]) {
            ans++;
            for (int k = j + 1; k < j + t + 1; k++) cnt[k]++;
            c[j] = 1;
            break;
          }
        }
      }
      if (cnt[i] < r) {
        cout << -1;
        return 0;
      }
    }
  }
  cout << ans;
  return 0;
}
