#include <bits/stdc++.h>
using namespace std;
int n, t, r, x, A[2000], B[2000], CODE = 0, ans = 0, z, j;
void Init() {
  cin >> n >> t >> r;
  if (r > t)
    CODE = 1;
  else
    for (int i = 0; i < 1000; i++) {
      A[i] = 0;
      B[i] = 0;
    }
}
int main() {
  Init();
  if (CODE == 1) {
    for (int i = 0; i < n; i++) cin >> x;
    cout << -1;
  } else {
    for (int i = 0; i < n; i++) {
      cin >> x;
      x += 500;
      if (B[x] < r) {
        j = x - 1;
        z = r - B[x];
        while (z != 0) {
          if (A[j] == 0) {
            A[j] = 1;
            for (int q = j + 1; q < j + t + 1; q++) B[q]++;
            ans++;
            z--;
          }
          j--;
        }
      }
    }
    cout << ans;
  }
  return 0;
}
