#include <bits/stdc++.h>
using namespace std;
int can[305];
int main() {
  int u, i, j, m, t, r, x, ret;
  cin >> m >> t >> r;
  ret = 0;
  if (t < r) {
    printf("-1\n");
    return 0;
  }
  for (i = 0; i < 300; ++i) can[i] = -1e9;
  for (i = 0; i < m; ++i) {
    scanf("%d", &x);
    u = x;
    for (j = 0; j < r; ++j)
      if (can[j] + t < x + 1) can[j] = u--, ++ret;
    sort(can, can + r);
  }
  cout << ret << endl;
}
