#include <bits/stdc++.h>
using namespace std;
const int N = 320;
int a[3 * N], b[3 * N];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int m, r, t, x, ans = 0;
  cin >> m >> t >> r;
  if (t < r) return cout << -1, 0;
  for (int i = 0; i < m; i++) {
    cin >> x;
    x += N;
    int now = x - 1;
    while (a[x] < r) {
      if (b[now] == 0) {
        ans++;
        b[now] = 1;
        for (int j = 0; j < t; j++) a[now + 1 + j]++;
      }
      now--;
    }
  }
  cout << ans;
  return 0;
}
