#include<set>
#include<map>
#include<stack>
#include<queue>
#include<cmath>
#include<cctype>
#include<cstdio>
#include<string>
#include<vector>
#include<cstring>
#include<cstdlib>
#include<utility>
#include<iostream>
#include<algorithm>
using namespace std;
typedef long long LL;
typedef pair<int, int> pii;
const int INF = 0x3f3f3f3f;
string s, sk;
int main()
{
  freopen("JOI.in", "r", stdin);
  freopen("JOI.out", "w", stdout);
  int k; cin >> k;
  for(int i = k; i >= 1; i--){
    sk.append(1 << ((i - 1) << 1), 'J');
    sk.append(1 << ((i - 1) << 1), 'O');
    sk.append(1 << ((i - 1) << 1), 'I');
  }
  cin >> s;
  int ans = s.size();
  for(int i = 0; i < s.size(); i++){
    int dif = 0;
    for(int j = 0; j < sk.size(); j++)
      if(sk[j] != s[(i + j) % s.size()]) dif++;
    if(dif < ans) ans = dif;
  }
  cout << ans << endl;
  return 0;
}