#include <bits/stdc++.h>
using namespace std;
template <class T>
T nrem(T x, T y) {
  return ((x % y) + y) % y;
}
void in() {}
int dp[605];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  in();
  int m, t, r;
  cin >> m >> t >> r;
  int g;
  if (t < r) return cout << -1 << "\n", 0;
  int ans = 0;
  while (m--) {
    cin >> g;
    int rem = r - dp[g];
    ans += rem;
    for (int i = 1; i <= rem; ++i)
      for (int j = 1; j <= t; ++j)
        if (g - i + j >= 0) dp[g - i + j]++;
  }
  cout << ans << "\n";
}
