#include <fstream>
#include <algorithm>
#include <string>
#define N 100010
using namespace std;
int K,n;
int ans=0,sum=0,INF=(1<<28);
char C[N],D[N],S[N];
ifstream in("JOI.in");
ofstream out("JOI.out");
void read()
{
	int i,j,k,cnt=0,temp;
	in>>K;
	n=1<<(2*K);
	ans=INF;
	for(i=1;i<=n;i++)in>>C[i];
	for(k=K;k>=1;k--)//生成标准序列
	{
		temp=(1<<(2*k-2));
		for(i=1;i<=3;i++)
		{
			for(j=1;j<=temp;j++)
			{
				cnt++;
				if(i==1)S[cnt]='J';
				if(i==2)S[cnt]='O';
				if(i==3)S[cnt]='I';
			}
		}
	}
	for(i=1;i<=n;i++)//枚举断点
	{
		cnt=sum=0;
		for(j=i+1;j<=n;j++)D[++cnt]=C[j];
		for(j=1;j<=i;j++)D[++cnt]=C[j];
		for(j=1;j<n;j++)if(D[j]!=S[j])sum++;
		if(ans>sum)ans=sum;
	}
	out<<ans<<endl;
}
int main()
{
	read();
	return 0;
}