#include <algorithm>
#include <array>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
int solve(long long n, 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(),0))/n;
}
int main(){
	long long n; string s;
	cin >> n >> s;
	reverse(s.begin(), s.end());
	cout << n-solve(n, s) << "\n";
}