#include <bits/stdc++.h>
using namespace std;
//#include <atcoder/all>
//using namespace atcoder;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvvb = vector<vvb>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
template<class T>
bool chmax(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

template<class T>
bool chmin(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

ll modPow(long long a, long long n, long long p) {
    if (n == 0) return 1; // 0乗にも対応する場合
    if (n == 1) return a % p;
    if (n % 2 == 1) return (a * modPow(a, n - 1, p)) % p;
    long long t = modPow(a, n / 2, p);
    return (t * t) % p;
}


ll gcd(ll(a), ll(b)) {
    if (a == 0)return b;
    if (b == 0)return a;
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}
ll sqrtz(ll N) {
    ll L = 0;
    ll R = sqrt(N) + 10000;
    while (abs(R - L) > 1) {
        ll mid = (R + L) / 2;
        if (mid * mid <= N)L = mid;
        else R = mid;
    }
    return L;

}
using pq=priority_queue<pair<ll,ll>,vector<pair<ll,ll>>,greater<pair<ll,ll>>>;


int main() {

    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll N;
    string S;
    cin>>N>>S;

    ll an=1e18;
    string T="spbsu";
    string G="spbsu";
    ll te=0;
        string C=T;
        rep(i,5){
            for(ll j=i;j<5;j++){
                if(C[j]==G[i]){
                    for(ll k=j-1;k>=i;k--){
                        swap(C[k],C[k+1]);
                        te++;
                    }
                    break;
                }
            }
        }

        vvll DP(N+1,vll(6,1e17));
        DP[0][0]=0;
        rep(i,N){
            rep(k,6){
                chmin(DP[i+1][k],DP[i][k]);
                if(k!=5){
                    if(T[k]==S[i]){
                        if(k==0||k==1){
                            chmin(DP[i+1][k+1],DP[i][k]-i);
                        }
                        else if(k==2){
                            chmin(DP[i+1][k+1],DP[i][k]);
                        }
                        else{
                            chmin(DP[i+1][k+1],DP[i][k]+i);
                        }
                    }
                }
            }
        }
        if(DP[N][5]<1e16)chmin(an,DP[N][5]+te-6);
    cout<<an<<endl;
    
}