#include <bits/stdc++.h>
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long int exp(long long int x, long long int y) {
  long long int res = 1;
  x = x % 1000000007;
  while (y > 0) {
    if (y & 1) res = (res * x) % 1000000007;
    y = y >> 1;
    x = (x * x) % 1000000007;
  }
  return res;
}
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long int m, t, r, c, i, j, ans = 0;
  cin >> m >> t >> r;
  vector<long long int> v(m);
  map<long long int, long long int> ma;
  for (long long int i = 0; i < m; i++) cin >> v[i];
  if (r > t)
    cout << -1;
  else {
    for (i = 0; i < m; i++) {
      c = 0;
      for (j = v[i] - 1; j >= v[i] - t; j--) {
        if (ma[j]) c++;
      }
      if (c != r) {
        j = v[i] - 1;
        while ((r - c) != 0) {
          if (!ma[j]) {
            ma[j] = 1;
            c++;
          }
          j--;
        }
      }
    }
    for (map<long long int, long long int>::iterator it = ma.begin();
         it != ma.end(); it++) {
      if (it->second == 1) ans++;
    }
    cout << ans;
  }
  return 0;
}
