#include "bits/stdc++.h"
using namespace std;
#ifdef DEBUG
auto&operator<<(auto &o, pair<auto, auto> p) {o << "(" << p.first << ", " << p.second << ")"; return o;}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{"; for(auto e : x) o<<e<<", "; return o<<"}";}
#define debug(X) cerr << "["#X"]: " << X << '\n';
#else
#define cerr if(0)cout
#define debug(X) ;
#endif
using ll = long long;
#define all(v) (v).begin(), (v).end()
#define ssize(x) int(x.size())
#define fi first
#define se second
#define mp make_pair
#define eb emplace_back
bool sorted(const vector<int> &a) {
for (int i = 0; i < ssize(a) - 1; ++i)
if (a[i] >= a[i+1]+2) return false;
return true;
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n;
cin >> n;
vector<int> h(n);
for (int &x : h) cin >> x;
map<vector<int>, int> dist;
queue<vector<int>> q;
dist[h] = 1;
q.push(h);
while (ssize(q)) {
vector<int> x = q.front();
if (sorted(x)) {
debug(x);
cout << dist[x] - 1 << '\n';
return 0;
}
int d = dist[x];
q.pop();
for (int i = 0; i < n-1; ++i) {
swap(x[i], x[i+1]);
if (dist[x] == 0) {
dist[x] = d + 1;
q.push(x);
}
swap(x[i], x[i+1]);
}
}
return 0;
}
