#include <algorithm>
#include <array>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
int solve(long long n, const string& s) {
  array<long long, 3> c;
  for (int i = 0; i < 3; ++i) c[i] = count(s.begin(), s.end(), 'A' + i);
  long long total_good = c[0] * c[0] + c[1] * c[1] + c[2] * c[2];
  partial_sum(c.begin(), c.end(), c.begin());
  vector<long long> v(n);
  for (int i = 0; i < n; ++i) {
    ++v[(i + n - c[(s[i] - 'A' + 0) % 3]) % n];
    --v[(i + n - c[(s[i] - 'A' + 2) % 3]) % n];
  }
  partial_sum(v.begin(), v.end(), v.begin());
  return *max_element(v.begin(), v.end()) +
         (total_good - accumulate(v.begin(), v.end(), 0L)) / n;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  long long n;
  string s;
  cin >> n >> s;
  int a = solve(n, s);
  reverse(s.begin(), s.end());
  int b = solve(n, s);
  cout << n - max(a, b) << "\n";
}