#include <bits/stdc++.h>
using namespace std;
int a[310], b[310];
int main() {
  int m, t, r;
  cin >> m >> t >> r;
  for (int i = 0; i < m; i++) cin >> a[i];
  if (t < r)
    cout << -1 << endl;
  else {
    int ans = 0;
    for (int i = 0; i < m; i++) {
      for (int j = a[i] - r + b[a[i]]; j < a[i]; j++) {
        ans++;
        for (int k = max(j + 1, 0); k <= min(j + t, 305); k++) b[k]++;
      }
    }
    cout << ans << endl;
  }
  return 0;
}
