#include<bits/stdc++.h>

using namespace std;

typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;

#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define X first
#define Y second
#define pb push_back
#define int long long

const LL mod = 1e9 + 7;


template<class T>
istream& operator >> (istream& in, vector<T>& v){ for (auto &x : v) { in >> x; } return in; }

template<class T>
istream& operator >> (istream& in, pair<T, T> & v){ in >> v.X >> v.Y;return in; }

template<class T>
ostream& operator << (ostream& out, pair<T, T> & v){ out << v.X << " " << v.Y;return out; }

void chkmax(int &a, int b) {
    a = max(a, b);
    return;
}

void chkmin(int &a, int b) {
    a = min(a, b);
    return;
}

LL ppow (LL x, LL s) {
    if (!s) return 1;
    if (!(s - 1)) return x % mod;
    if (s % 2) return (x * ppow (x, s - 1)) % mod;
    LL b = ppow (x, s / 2);
    return (b * b) % mod;
}

main(){
    //ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    int n;
    cin >> n;
    vector<int> a(n);
    cin >> a;
    vector<int> pos[25];
    for (int i = 0;i < n;i++) {
        pos[--a[i]].pb(i);
    }
    int cnt[21][21];
    for (int i = 0;i < 20;i++) {
        for (int j = 0;j < 20;j++) {
            cnt[i][j] = 0;
            if (i == j) {
                continue;
            }
            if (pos[i].size() == 0 || pos[j].size() == 0) {
                continue;
            }
            for (auto x : pos[i]) {
                int c = int32_t(upper_bound(all(pos[j]), x) - pos[j].begin());
                cnt[i][j] += c;
            }
        }
    }
    vector<int> dp((1 << 20), 1e18);
    dp[0] = 0;
    for (int mask = 0;mask < (1 << 20);mask++) {
        for (int i = 0;i < 20;i++) {
            int cur = 0;
            if (mask&(1 << i)) {
                continue;
            }
            for (int j = 0;j < 20;j++) {
                if (mask&(1 << j)) {
                    cur += cnt[j][i];
                }
            }
            chkmin(dp[mask|(1 << i)], dp[mask] + cur);
        }
    }
    /*for (int i = 0;i < (1 << 3);i++) {
        cout << dp[i] << '\n';
    }*/
    cout << dp[(1 << 20) - 1];
    return 0;
}