#include <bits/stdc++.h>
using namespace std;
int l;
string a, b;
int ans = 0x3f3f3f;
int main() {
  ios::sync_with_stdio(false);
  cin >> l, cin >> a, b = a;
  sort(a.begin(), a.end());
  for (int i = 0, key = 0, cnt = 0; i < l;
       i++, ans = min(ans, cnt / 2), key = i, cnt = 0)
    for (int j = 0; j < l; j++, key = (key + 1) % l)
      if (b[j] != a[key]) cnt++;
  printf("%d", ans);
  return 0;
}
