#include <bits/stdc++.h>

using i64 = long long;
constexpr int inf = 1e9;

int main() {
    std::cin.tie(nullptr)->sync_with_stdio(false);

    std::string msg = "spbsu";

    int n;
    std::cin >> n;
    
    std::string s;
    std::cin >> s;

    auto work = [&]() {
        std::vector<std::array<int, 6>> f(n + 1);
        f[0].fill(inf), f[0][0] = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 1; j < 6; ++j) {
                f[i + 1][j] = std::min(f[i][j] + j, inf);
                if (s[i] == msg[j - 1]) {
                    f[i + 1][j] = std::min(f[i + 1][j], f[i][j - 1]);
                }
            }
        }
        return f;
    };

    auto f = work();
    std::reverse(s.begin(), s.end());
    std::reverse(msg.begin(), msg.end());
    auto g = work();

    int ans = inf;
    for (int i = 0; i < n; ++i) {
        for (int j = 1; j < 6; ++j) {
            if (s[i] == msg[j - 1]) {
                ans = std::min(ans, g[i][j - 1] + f[n - i - 1][5 - j]);
            }
        }
    }

    std::cout << ans << '\n';

    return 0;
}