#include <bits/stdc++.h>
using namespace std;

const int N = 1 << 20;

int n, k, ans, v[N], cnt[N][11][3], f[N][11];

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> k;
	n = 1 << k * 2;
	for (int i = 0; i < n; i++) {
		char c;
		cin >> c;
		if (c == 'J') {
			v[i] = 0;
		} else if (c == 'O') {
			v[i] = 1;
		} else {
			v[i] = 2;
		}
	}
	for (int i = 0; i < n; i++) {
		cnt[i][0][v[i]] = 1;
		f[i][0] = 1;
	}
	for (int j = 1; j <= k; j++) {
		for (int i = 0; i < n; i++) {
			int step = 1 << (j - 1) * 2;
			int i1 = (i + step) % n;
			int i2 = (i1 + step) % n;
			int i3 = (i2 + step) % n;
			for (int t = 0; t < 3; t++) {
				cnt[i][j][t] = cnt[i][j - 1][t] + cnt[i1][j - 1][t] + cnt[i2][j - 1][t] + cnt[i3][j - 1][t];
			}
			f[i][j] = cnt[i][j - 1][0] + cnt[i1][j - 1][1] + cnt[i2][j - 1][2] + f[i3][j - 1];
		}
	}
	for (int i = 0; i < n; i++) {
		ans = max(ans, f[i][k]);
	}
	cout << n - ans << '\n';

	return 0;
}