#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, t, r, el[305];
  scanf("%d%d%d", &m, &t, &r);
  if (t < r) {
    printf("-1");
    return 0;
  }
  for (int i = 1; i <= r; ++i) el[i] = -1;
  int ans = 0;
  for (int i = 1; i <= m; ++i) {
    int x;
    scanf("%d", &x);
    int k = t - 1;
    for (int j = 1; j <= r; ++j) {
      if (x > el[j]) {
        el[j] = x + k;
        --k;
        ++ans;
      }
    }
  }
  printf("%d", ans);
  return 0;
}
