#include <bits/stdc++.h>
using namespace std;
const int MaxN = 1e6;
int n, t, r, w[MaxN + 5], ans, a[MaxN + 5];
int main() {
  scanf("%d%d%d", &n, &t, &r);
  for (int i = 1; i <= n; i++) scanf("%d", &w[i]);
  sort(w + 1, w + n + 1);
  if (t - r < 0)
    printf("-1");
  else {
    for (int i = 1; i <= n; i++) {
      int x = w[i] - 1;
      while (a[w[i] + 1000] < r) {
        for (int k = x - 1; k <= x + t; k++) a[k + 1000]++;
        ans++;
        x--;
      }
    }
    printf("%d", ans);
  }
}
