#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define ertunt return
const int mod = 998244353;
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
ll n;
cin >> n;
string s, t;
cin >> s;
t = s;
sort(all(s));
ll ans = 1e18;
do {
bool k = false;
for (ll i = 1; i < n; i++) {
if (s[i] == s[i - 1]) {
k = true;
break;
}
}
if (k) continue;
ll res = 0;
string u = s;
for (ll i = 0; i < n; i++) {
for (ll j = i; j < n; j++) {
if (t[i] == u[j]) {
for (ll k = j; k > i; k--) {
swap(u[k], u[k - 1]);
res++;
}
break;
}
}
}
ans = min(ans, res);
} while (next_permutation(all(s)));
cout << (ans == 1e18 ? -1 : ans);
}
