//#pragma GCC optimize("Ofast", "unroll-loops")
//#pragma GCC target("sse", "sse2", "sse3", "ssse3", "sse4")
#ifdef LOCAL
#include <iostream>
#include <cmath>
#include <algorithm>
#include <stdio.h>
#include <cstdint>
#include <cstring>
#include <string>
#include <cstdlib>
#include <vector>
#include <bitset>
#include <map>
#include <queue>
#include <ctime>
#include <stack>
#include <set>
#include <list>
#include <random>
#include <deque>
#include <functional>
#include <iomanip>
#include <sstream>
#include <fstream>
#include <complex>
#include <numeric>
#include <cassert>
#include <array>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <thread>
#else
#include <bits/stdc++.h>
#endif

#define all(a) a.begin(),a.end()
#define len(a) (int)(a.size())
#define mp make_pair
#define pb push_back
#define fir first
#define sec second
#define fi first
#define se second

using namespace std;

typedef pair<int, int> pii;
typedef long long ll;
typedef long double ld;

template<typename T>
inline bool umin(T &a, T b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}

template<typename T>
inline bool umax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

#ifdef LOCAL
#define D for (bool _FLAG = true; _FLAG; _FLAG = false)
#define LOG(...) print(#__VA_ARGS__" ::", __VA_ARGS__) << endl
template <class ...Ts> auto &print(Ts ...ts) { return ((cerr << ts << " "), ...); }
#else
#define D while (false)
#define LOG(...)
#endif // LOCAL

const int max_n = -1, inf = 1000111222;


inline int sum (int x) {
    if (x <= 1) {
        return 0;
    }
    --x;
    return x * (x + 1) / 2;
}


int main() {
//    freopen("input.txt", "r", stdin);
//    freopen("output.txt", "w", stdout);

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

    int n;
    cin >> n;
    string s;
    cin >> s;
    LOG(n);
    int ans = inf;
    string l = "spbsu";
    vector <vector <int> > dp(n + 1, vector <int> (6, -inf));
    vector <vector <int> > rev(n + 1, vector <int> (6, inf));
    dp[0][0] = 0;
    rev[n][0] = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < 6; j++) {
            umax(dp[i + 1][j], dp[i][j]);
            if (j < len(l) && s[i] == l[j]) {
                umax(dp[i + 1][j + 1], dp[i][j] + i);
            }
        }
    }
    reverse(all(l));
    for (int i = n - 1; i >= 0; i--) {
        for (int j = 0; j < 6; j++) {
            umin(rev[i][j], rev[i + 1][j]);
            if (j < len(l) && s[i] == l[j]) {
                umin(rev[i][j + 1], rev[i + 1][j] + i);
            }
        }
    }
    for (int i = 0; i < n; i++) {
        for (int lft = 0; lft <= len(l); lft++) {
            int L = dp[i + 1][lft];
            int R = rev[i + 1][len(l) - lft];
            if (L < 0 || R >= inf) {
                continue;
            }
            int cur = i * lft - L + (R - (len(l) - lft) * (i + 1)) - sum(lft) - sum(len(l) - lft);
            LOG(i, L, R, cur, i * lft - L - sum(lft), lft, (R - (len(l) - lft) * (i + 1)));
            umin(ans, cur);
        }
    }
    cout << ans << '\n';
}