#include <bits/stdc++.h>
using namespace std;

template <class T, class U = T> bool chmin(T& x, U&& y) { return y < x and (x = forward<U>(y), true); }

constexpr int INF = (1 << 30) - 1;
const string target = "spbsu";

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    string S;
    cin >> n >> S;

    vector<vector<int>> pos(5);
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < 5; j++) {
            if (S[i] == target[j]) {
                pos[j].emplace_back(i);
            }
        }
    }
    int ans = INF;
    vector<int> cur(5, 0), len(5);
    for (int i = 0; i < 5; i++) len[i] = pos[i].size();
    for (int i = 0; i < len[2]; i++) {
        while (cur[1] + 1 < len[1] and pos[1][cur[1] + 1] < pos[2][i]) cur[1]++;
        while (cur[0] + 1 < len[0] and pos[0][cur[0] + 1] < pos[1][cur[1]]) cur[0]++;
        while (cur[3] < len[3] and pos[3][cur[3]] < pos[2][i]) cur[3]++;
        if (cur[3] == len[3]) break;
        while (cur[4] < len[4] and pos[4][cur[4]] < pos[3][cur[3]]) cur[4]++;
        if (cur[4] == len[4]) break;
        if (pos[2][i] < pos[1][cur[1]]) continue;
        if (pos[1][cur[1]] < pos[0][cur[0]]) continue;
        vector<int> cnt(4);
        for (int i = 0; i < 4; i++) cnt[i] = pos[i + 1][cur[i + 1]] - pos[i][cur[i]] - 1;
        chmin(ans, cnt[0] + 2 * cnt[1] + 2 * cnt[2] + cnt[3]);
    }

    cout << ans << '\n';
}