oaksssskoa

//Includes of headers
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cassert>
#include <algorithm>
#include <deque>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <functional>
#include <sstream>
#include <numeric>
#include <random>
#include <chrono>

//Macro definations
#define all(C) (C).begin(), (C).end()
#define rall(C) (C).rbegin(), (C).rend()
#define mp make_pair
#define pb emplace_back
#define int ll
#define forn(i, n) for (int i = 0; i < (int)n; ++i)

using namespace std;

//C++11-style typedefs
using ll = long long;
using ull = unsigned long long;
using pii = pair <int, int>;
using pll = pair <ll, ll>;
using ld = long double;
using pld = pair <ld, ld>;
/*
 //Fast allocation
 const int MAX_MEM = 1e8;
 char mem[MAX_MEM];
 int mpos = 0;
 inline void * operator new (size_t n)
 {
 char *res = mem + mpos;
 mpos += n;
 assert(mpos <= MAX_MEM);
 return (void*) res;
 }
 inline void operator delete(void*) {}
*/
//IO-streams templates
template <class F, class S> istream &operator>> (istream &in, pair <F, S> &a)
{
    in >> a.first >> a.second;
    return in;
}
template <class F, class S> ostream &operator<< (ostream &out, pair <F, S> &a)
{
    out << a.first << " " << a.second;
    return out;
}
template <class T> istream &operator>> (istream &in, vector <T> &a)
{
    for (auto &i : a)
        in >> i;
    return in;
}
template <class T> ostream &operator<< (ostream &out, vector <T> &a)
{
    for (auto &i : a)
        out << i << " ";
    return out;
}

//Fast IO
inline void Start()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    //freopen("points.in", "r", stdin);
    //freopen("points.out", "w", stdout);
}

const int INF = 1e9;

signed main()
{
    Start();
    int n;
    cin >> n;
    vector <int> a(n);
    cin >> a;
    vector <vector <bool>> del(n, vector <bool> (n, 0));
    for (int i = 0; i < n; ++i)
    {
        del[i][i] = true;
        if (i)
            del[i - 1][i] = true;
    }
    for (int len = 1; len < n; ++len)
    {
        for (int l = 0; l + len < n; ++l)
        {
            int r = l + len;
            for (int k = l + 1; k < r; ++k)
                del[l][r] = del[l][r] || (del[l][k - 1] && del[k + 1][r] && (a[l] < a[k] && a[k] > a[r] || a[l] > a[k] && a[k] < a[r]));
        }
    }
    vector <vector <int>> dp(n, vector <int> (n, 0));
    for (int len = 1; len < n; ++len)
    {
        for (int l = 0; l + len < n; ++l)
        {
            int r = l + len;
            dp[l][r] = INF;
            if (a[r] < a[l])
                continue;
            if (l + 1 < n && r - 1 >= 0)
                dp[l][r] = min(dp[l][r], dp[l + 1][r - 1]);
            for (int k = l + 1; k < r; ++k)
            {
                if (a[k - 1] <= a[k + 1] && (a[k - 1] > k && k < a[k + 1] || a[k - 1] < k && k > a[k + 1]))
                    dp[l][r] = min(dp[l][r], dp[l][k - 1] + dp[k + 1][r] + 1);
            }
            if (del[l][r])
                dp[l][r] = min(dp[l][r], r - l - 1);
        }
    }
    if (dp[0][n - 1] == INF)
    {
        cout << -1;
        return 0;
    }
    cout << dp[0][n - 1];
    return 0;
}