#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, t, r;
  int num;
  bool vis[1009];
  int flag[1009], a[309];
  while (cin >> m >> t >> r) {
    num = 0;
    memset(vis, 0, sizeof(vis));
    memset(flag, 0, sizeof(flag));
    for (int i = 1; i <= m; i++) {
      cin >> a[i];
      a[i] += 300;
    }
    for (int i = 1; i <= m; i++) {
      if (flag[a[i]] < r) {
        for (int j = a[i] - 1; j >= a[i] - t; j--) {
          if (!vis[j]) {
            num++;
            vis[j] = 1;
            for (int k = j + 1; k <= j + t; k++) flag[k]++;
          }
          if (flag[a[i]] >= r) break;
        }
      }
      if (flag[a[i]] < r) {
        num = -1;
        break;
      }
    }
    cout << num << endl;
  }
}
