#include <bits/stdc++.h>
using namespace std;
int n, t, c, ans;
int a[333], k[1000];
int main(int argc, char **argv) {
  scanf("%d %d %d", &n, &t, &c);
  if (c > t) return puts("-1"), 0;
  for (int i = 0, x; i < n; ++i) {
    scanf("%d", &x);
    a[i] = x + 300;
  }
  for (int i = 0; i < n; ++i) {
    int tmp = 0;
    for (int j = a[i] - 1; j >= a[i] - t; --j)
      if (k[j]) ++tmp;
    if (tmp < c) {
      tmp = c - tmp;
      for (int j = a[i] - 1; j >= a[i] - c && tmp; --j)
        if (!k[j]) k[j] = 1, ++ans, --tmp;
    }
  }
  cout << ans << endl;
  return 0;
}
