#include <algorithm>
#include <array>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
int main(){
	long long n; string s;
	cin >> n >> s;
	array<vector<bool>, 3> mask;
	array<long long, 3> c, d;
	c.fill(0);
	for(int i = 0; i < 3; ++i){
		mask[i].resize(2*n);
		for(int j = 0; j < n; ++j)
			mask[i][j] = mask[i][j+n] = s[j] != 'A' + i, c[i] += 1-mask[i][j];
	}

	d[0] = c[0];
	d[1] = d[0] + c[1];
	d[2] = d[1] + c[2];

	int ans = n;
	for(int o = 0; o < n; ++o){
		int a = 0;
		int j = o;
		for(int i = 0; i < 3; ++i){
			for(; j < o+d[i]; ++j)
				a += mask[i][j];
		}
		ans = min(ans, a);
	}

	d[0] = c[2];
	d[1] = d[0] + c[1];
	d[2] = d[1] + c[0];

	for(int o = 0; o < n; ++o){
		int a = 0;
		int j = o;
		for(int i = 0; i < 3; ++i){
			for(; j < o+d[i]; ++j)
				a += mask[2-i][j];
		}
		ans = min(ans, a);
	}

	cout << ans << "\n";
}