#include <bits/stdc++.h>
using namespace std;
int Set(int N, int pos) { return N |= (1LL << pos); }
int Reset(int N, int pos) { return N &= ~(1LL << pos); }
bool Check(int N, int pos) { return N & (1LL << pos); }
int dx[] = {0, 0, 1, -1, 1, -1, 1, -1}, dy[] = {1, -1, 0, 0, 1, -1, -1, 1};
inline void cn(long &n) {
  n = 0;
  long ch = getchar();
  int sign = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') sign = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    n = (n << 3) + (n << 1) + ch - '0', ch = getchar();
  n = n * sign;
}
template <class T>
void cmin(T &a, T b) {
  if (b < a) a = b;
}
template <class T>
void cmax(T &a, T b) {
  if (b > a) a = b;
}
template <class T>
int len(const T &c) {
  return (int)c.size();
}
template <class T>
int len(char c[]) {
  return (int)strlen(c);
}
long stoi(string s) {
  long n = 0;
  for (int i(0), _n(len(s)); i < _n; ++i) n = n * 10 + (s[i] - 48);
  return n;
}
int main() {
  ios_base::sync_with_stdio(false);
  long m, t, r, w, k, ww, res = 0, ls;
  cin >> m >> t >> r;
  vector<int> v, tem;
  int f = 1;
  for (int i(0), _n(m); i < _n; ++i) {
    cin >> w;
    ls = -100000000000000000LL;
    if (!f) continue;
    for (int j = len(v) - 1; j >= 0; j--) {
      if (v[j] >= w)
        tem.push_back(v[j]);
      else
        break;
    }
    if (len(tem) < r) {
      k = r - len(tem);
      ww = w;
      for (int j(0), _n(k); j < _n; ++j) {
        if (ww + t - 1 >= w) {
          res++;
          tem.push_back(ww + t - 1);
          ww -= 1;
        } else {
          f = 0;
          break;
        }
      }
    }
    v = tem;
    tem.clear();
    sort(v.begin(), v.end());
  }
  if (f == 0) res = -1;
  cout << res;
  return 0;
}
