#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void remax(T &A, T B) {
  if (A < B) A = B;
}
template <class T>
inline void remin(T &A, T B) {
  if (A > B) A = B;
}
string ToString(long long num) {
  string ret;
  do {
    ret += ((num % 10) + '0');
    num /= 10;
  } while (num);
  reverse(ret.begin(), ret.end());
  return ret;
}
long long ToNumber(string s) {
  long long r = 0, p = 1;
  for (int i = s.size() - 1; i >= 0; --i) r += (s[i] - '0') * p, p *= 10;
  return r;
}
long long Gcd(long long a, long long b) {
  while (a %= b ^= a ^= b ^= a)
    ;
  return b;
}
long long Power(long long base, long long power) {
  long long ret = 1;
  while (power) {
    if (power & 1) ret *= base;
    power >>= 1;
    base *= base;
  }
  return ret;
}
long long PowerMod(long long base, long long power, long long mod) {
  if (!power) return 1;
  if (power & 1) return (base * PowerMod(base, power - 1, mod)) % mod;
  return PowerMod((base * base) % mod, power >> 1, mod);
}
int Log(long long num, long long base) {
  int ret = 0;
  while (num) {
    ++ret;
    num /= base;
  }
  return ret;
}
int Count(long long mask) {
  int ret = 0;
  while (mask) {
    if (mask & 1) ++ret;
    mask >>= 1;
  }
  return ret;
}
int memo[1000][1001][3];
int arr[1000];
int n;
int f(int ind, int c, int u) {
  if (ind == ~0 || ind == n) return 0;
  int &ret = memo[ind][c][u];
  if (~ret) return ret;
  ret = 1000000000;
  if (arr[ind] <= c)
    if (!u)
      ret = min(f(ind - 1, c + 1, 1), f(ind + 1, c + 1, 2));
    else if (u == 1)
      ret = min(f(ind - 1, c + 1, 1), f(ind + 1, c + 1, 2) + 1);
    else
      ret = min(f(ind - 1, c + 1, 1) + 1, f(ind + 1, c + 1, 2));
  else if (!u)
    ret = min(f(ind - 1, c, 1), f(ind + 1, c, 2));
  else if (u == 1)
    ret = min(f(ind - 1, c, 1), f(ind + 1, c, 2) + 1);
  else
    ret = min(f(ind - 1, c, 1) + 1, f(ind + 1, c, 2));
  return ret;
}
inline void run() {
  bool vis[1000] = {};
  int ans1 = 0, ans2 = 0, s = 0, tmp = 0, rem;
  bool ok = false;
  scanf("%d", &n);
  for (int i = 0; i < n;) scanf("%d", arr + i++);
  rem = n;
  while (rem) {
    if (!ok)
      for (int i = tmp; i < n; ++i) {
        if (arr[i] <= s && !vis[i]) {
          vis[i] = true;
          --rem;
          ++s;
          tmp = i;
        }
      }
    else
      for (int i = tmp; ~i; --i)
        if (arr[i] <= s && !vis[i]) {
          vis[i] = true;
          --rem;
          ++s;
          tmp = i;
        }
    if (rem) ++ans1;
    ok = !ok;
  }
  printf("%d\n", ans1);
}
int main() {
  FILE *input = stdin;
  FILE *output = stdout;
  while (!feof(input)) {
    run();
    break;
  }
  return 0;
}
