#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=300005,INF=1<<28;

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    string S;cin>>S;
    string U="spbsu",Z=U;
    
    sort(all(Z));
    
    map<string,int> MA;
    
    queue<string> Q;
    Q.push(U);
    MA[U]=0;
    
    while(!Q.empty()){
        auto S=Q.front();Q.pop();
        int d=MA[S];
        for(int i=0;i+1<5;i++){
            swap(S[i],S[i+1]);
            if(!MA.count(S)){
                MA[S]=d+1;
                Q.push(S);
            }
            swap(S[i],S[i+1]);
        }
    }
    
    int ans=INF;
    
    do{
        if(Z!=U) continue;
        vector<vector<int>> pos(5);
        for(int i=0;i<5;i++){
            for(int j=0;j<N;j++){
                if(Z[i]==S[j]) pos[i].push_back(j);
            }
        }
        int mi=INF;
        
        for(int c:pos[2]){
            int a,b,d,e;
            auto it=lower_bound(all(pos[1]),c);
            if(it==pos[1].begin()) continue;
            it--;
            b=(*it);
            
            it=lower_bound(all(pos[0]),b);
            if(it==pos[0].begin()) continue;
            it--;
            a=(*it);
            
            it=upper_bound(all(pos[3]),c);
            if(it==pos[3].end()) continue;
            d=(*it);
            
            it=upper_bound(all(pos[4]),d);
            if(it==pos[4].end()) continue;
            e=(*it);
            
            chmin(mi,(c-b-1)+(c-a-2)+(d-c-1)+(e-c-2));
        }
        
        chmin(ans,mi+MA[Z]);
        
    }while(next_permutation(all(Z)));
    
    cout<<ans<<endl;
}