#include <cstring>
#include <iostream>
#include <set>
using namespace std;
const int INF = 0x3f3f3f3f;
int a[500];
int dp2[500][501];
int solve(int i, int j) {
    if (i+1 == j) return 0;
    if (dp2[i][j] != -1) return dp2[i][j];
    int &res = dp2[i][j] = INF;
    for (int k=i+1; k < j; k++) {
        set<int> s1(a+i, a+k), s2(a+k, a+j);
        int steps = 0;
        set<int>::iterator it1=s1.begin(), it2=s2.begin();
        if (*it2 < *it1) {
            steps++;
            while(it2 != s2.end() && *it2 < *it1) it2++;
        }
        while(it1 != s1.end()) {
            steps++;
            while(it1 != s1.end() && it2 != s2.end() && *it1 < *it2) it1++;
            if (it2 == s2.end()) break;
            steps++;
            while(it1 != s1.end() && it2 != s2.end() && *it2 < *it1) it2++;
        }
        res = min(res, solve(i, k) + solve(k, j) + 2*steps-3);
    }
    return res;
}
int main() {
    int n; cin >> n;
    for (int i=0; i < n; i++) cin >> a[i];
    int dp[n+1]; dp[0] = 0; memset(dp2, -1, sizeof dp2);
    for (int i=1; i <= n; i++) {
        dp[i] = INF;
        for (int j=0; j < i; j++) {
            set<int> s(a+j, a+i);
            if (s.size() == i-j && *(--s.end()) == i-j)
                dp[i] = min(dp[i], dp[j] + solve(j, i));
        }
    }
    if (dp[n] == INF) cout << "impossible" << endl;
    else cout << dp[n] << endl;
    return 0;
}