#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int calc(int n, vector<int>& a, vector<int> b) {
int res = 0;
for (int i = 0; i < n; ++i) {
int id = -1;
for (int j = i; j < n; ++j) {
if (b[j] == a[i]) {
id = j;
break;
}
}
while (id > i) {
swap(b[id - 1], b[id]);
id -= 1;
res += 1;
}
}
assert(b == a);
return res;
}
int main() {
int n;
cin >> n;
vector<int> a(n);
iota(begin(a), end(a), 1);
vector<int> b(n);
for (auto& u : b) cin >> u;
int res = n * n;
do {
int mx = 0;
bool ok = true;
for (int i = 0; i < n; ++i) {
if (2 * i + a[i] <= mx) {
ok = false;
break;
}
mx = max(mx, 2 * i + a[i]);
}
if (ok) {
res = min(res, calc(n, b, a));
//~ cout << calc(n, a, b) << " = ";
//~ for (auto& u : a) {
//~ cout << u << " ";
//~ }
//~ cout << "\n";
}
} while (next_permutation(begin(a), end(a)));
cout << res << "\n";
}
