#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pii;
#define all(x) x.begin(), x.end()
#define sz(x) ((int) ((x).size()))
#define pb push_back
#define F first
#define S second
#define FIO ios_base::sync_with_stdio(false); cin.tie(0)
const int N = 200;
int n;
string s;
int st[N];
int run(vector<int> v) {
for (int i = 0; i < n; i++) {
if (v[v[i]] != i) return 0;
if (s[v[i]] == s[i]) return 0;
}
/*
cout << "CONF ";
for (int x : v) {
cout << x << " ";
}
cout << endl;*/
int ans = 0;
for (int i = 0; i < n; i++) {
if (i < v[i]) continue;
for (int j = 0; j < n; j++) {
st[j] = 0;
}
int cur = (v[i] + 1) % sz(v);
vector<int> k;
while (cur != i) {
k.pb(cur);
st[cur] = 1;
cur++;
cur %= sz(v);
}
for (int x : k) {
if (st[v[x]] == 0) {
ans++;
}
}
}
//cout << "ANS: " << ans << endl;
return ans;
}
int main() {
FIO;
cin >> n;
cin >> s;
n *= 2;
vector<int> v;
for (int i = 0; i < n;i ++) {
v.pb(i);
}
int ans = 0;
do {
ans = max(ans, run(v));
} while (next_permutation(all(v)));
cout << ans / 2 << endl;
return 0;
}
