#include <bits/stdc++.h>
using namespace std;
template <class T>
bool isPrime(T x) {
  if (x <= 1) return false;
  T i;
  for (i = 2; i * i <= x; i++)
    if (x % i == 0) return false;
  return true;
}
template <class T>
class Prime {
 public:
  vector<T> z;
  Prime() {
    z.resize(1e5 + 7);
    for (int i = (0); i < (((int)z.size())); i++) z[i] = 1;
    z[0] = 0;
    z[1] = 0;
    T j;
    for (int i = (2); i < (((int)z.size())); i++) {
      if (z[i]) {
        j = i + i;
        while (j < ((int)z.size())) {
          z[j] = 0;
          j += i;
        }
      }
    }
  }
};
template <class T>
double dist(T x1, T y1, T x2, T y2) {
  return sqrt(1. * (x2 - x1) * (x2 - x1) + 1. * (y2 - y1) * (y2 - y1));
}
template <class T>
class Point {
 public:
  T x, y;
  Point() {}
  Point(T a, T b) : x(a), y(b) {}
  bool operator==(const Point& tmp) const { return (x == tmp.x && y == tmp.y); }
  Point operator-(const Point& tmp) const {
    return Point<T>(x - tmp.x, y - tmp.y);
  }
};
char toLowerCase(char x) { return (x + 32); }
char toUpperCase(char x) { return (x - 32); }
bool isUpperCase(char x) { return (65 <= x && x <= 90) ? 1 : 0; }
bool isLowerCase(char x) { return (97 <= x && x <= 122) ? 1 : 0; }
bool isAlpha(char x) { return (isUpperCase(x) || isLowerCase(x)) ? 1 : 0; }
bool isDigit(char x) { return ('0' <= x && x <= '9') ? 1 : 0; }
template <class T>
T toDec(string s) {
  stringstream is(s);
  T res;
  is >> res;
  return res;
}
template <class T>
string toStr(T n) {
  string s;
  stringstream is;
  is << n;
  is >> s;
  return s;
}
template <class T>
void checkmin(T& a, T b) {
  if (a > b) a = b;
}
template <class T>
void checkmax(T& a, T b) {
  if (a < b) a = b;
}
int main() {
  int m, t, r;
  cin >> m >> t >> r;
  vector<int> was(m), used(611, 0);
  set<int> lt, rt;
  int res = r;
  int off = 300;
  bool possible = true;
  for (int i = 0; i < m; i++) cin >> was[i];
  for (int i = 0; i < r; i++) {
    used[was[0] - i - 1 + off] = 1;
    lt.insert(was[0] - i);
    rt.insert(was[0] - i + t - 1);
  }
  for (int i = 0; i < m; i++) {
    int rem = 0;
    vector<int> arr;
    for (typeof(rt.begin()) it = rt.begin(); it != rt.end(); it++) {
      int val = *it;
      if (val < was[i]) {
        rem++;
        arr.push_back(val);
        lt.erase(val - t + 1);
        used[val - t + off] = 0;
      }
    }
    res += rem;
    for (int j = 0; j < ((int)arr.size()); j++) rt.erase(arr[j]);
    int j = 0;
    while (rem) {
      int opt = was[i] - j - 1;
      if (!used[opt + off]) {
        if (was[i] - j + t - 1 < was[i]) {
          possible = false;
          break;
        } else {
          lt.insert(was[i] - j);
          rt.insert(was[i] - j + t - 1);
          used[opt + off] = 1;
          rem--;
        }
      }
      j++;
    }
    if (!possible) break;
  }
  cout << (possible ? res : -1);
  return 0;
}
