///#include "dungeon2.h"
#pragma GCC optimize("Ofast,unroll-loops")
#include<bits/stdc++.h>
//#define int   long long
#define ll long long
#define down cout<<'\n';
#define debug cout<<" cucuucucuuu",down
#define modwwe  int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task2 "ftree"
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define pb push_back
#define mask(k) (1<<k)
#define checktime   cerr << (double)clock() / CLOCKS_PER_SEC * 1000  << " ms";
using namespace std;
#define getchar_unlocked getchar
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
int rand(int l,int r)
{
    return uniform_int_distribution<int>(l,r)(rd);
}
void phongbeo();
const int inf = 1e9;
const ll mod2 = 1e9+7;
const ll base=67;
int  n, m, s1, s2, s4, s3, sf, k, s5, s6, mx, s7, s8, s9, mx2, res, dem2 = 0, dem = 0, s33, dem3, dem4, mid, l2, r2, center;
int  i, s10, s12,k1,k2,k3,s11,lim,w,l,r,dem5,dem6,dem7,dem9;
int kk;
int t;
int el = 19;
main()
{
    if(fopen(task2".inp","r"))
    {
        fin(task2);
        fou(task2);
    }
    if(fopen(task".inp","r"))
    {
        fin(task);
        fou(task);
    }
    ios_base::sync_with_stdio(0);
    cout.tie(0);
    cin.tie(0);
    /// cin>>s1;
//int t;cin>>t; while(t--)
    phongbeo();
    checktime
}
string s;
int a[mask(20)+1][3];
int dp[mask(20)+1][21];
int get(int x,int y,int z)
{
    if(x<=y)return (y-x+1)-(a[y][z]-a[x-1][z]);
    return (mask(n*2)-x+1+y)-(a[y][z]+a[mask(n*2)][z]-a[x-1][z]);
}
void phongbeo()
{
    cin>>n;
    cin>>s;
    for(int i=1; i<=mask(n*2); i++)
    {
        int x=0;
        if(s[i-1]=='O')x=1;
        if(s[i-1]=='I')x=2;
        a[i][x]++;
        for(int j=0; j<3; j++)
            a[i][j]+=a[i-1][j];
    }
    s4=inf;
    for(int j=1; j<=n; j++)
        for(int i=1; i<=mask(n*2); i++)
        {
            int i1=(i+mask(j*2-2)-1)%mask(n*2)+1;
            int i2=(i1+mask(j*2-2)-1)%mask(n*2)+1;
            int i3=(i2+mask(j*2-2)-1)%mask(n*2)+1;
            dp[i][j]=get(i,i1-1,0)+get(i1,i2-1,1)+get(i2,i3-1,2)+dp[i3][j-1];
            if(j==n)s4=min(s4,dp[i][j]);
        }
    cout<<s4;
}