#include <bits/stdc++.h>
using namespace std;
const int INF = INT_MAX / 2, NINF = INT_MIN / 2;
const long double PI = acos(-1.0);
void prepareIO() {
  if (strlen("") != 0) {
    freopen(
        ""
        ".in",
        "r", stdin);
    freopen(
        ""
        ".out",
        "w", stdout);
  }
}
void debout() {}
template <typename Head, typename... Tail>
void debout(Head H, Tail... T) {}
void stressout() {}
template <typename Head, typename... Tail>
void stressout(Head H, Tail... T) {}
void solve1();
int main() {
  prepareIO();
  solve1();
  fprintf(stderr, "%.6lf\n", (double)clock() / CLOCKS_PER_SEC);
  return 0;
}
struct T {
  int to, cnt, pos;
};
const int N = 1010;
int a[N], mark[N];
void solve1() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int ans = 0, cnt = 0;
  int to = 1;
  int pos = 0;
  while (cnt < n) {
    if (a[pos] <= cnt && mark[pos] == false) {
      mark[pos] = 1;
      cnt++;
      continue;
    }
    if (to == 1) {
      if (pos == n - 1) {
        ans++;
        to ^= 1;
        continue;
      }
      pos++;
    } else {
      if (pos == 0) {
        ans++;
        to ^= 1;
        continue;
      }
      pos--;
    }
  }
  printf("%d\n", ans);
}
