#include <bits/stdc++.h>

using i64 = long long;

constexpr int inf = 1E9;

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

    int n;
    std::cin >> n;

    std::string s;
    std::cin >> s;

    std::string t = "spbsu";

    std::array<int, 6> dp{};
    dp.fill(inf);
    dp[0] = 0;
    for (int i = 0; i < n; i++) {
        auto ndp = dp;
        for (int j = 0; j < 5; j++) {
            if (s[i] == t[j]) {
                int s = 0;
                if (j < 2) {
                    s = -1;
                } else if (j > 2) {
                    s = 1;
                }
                ndp[j + 1] = std::min(ndp[j + 1], dp[j] + s * i);
            }
        }
        dp = std::move(ndp);
    }
    int ans = dp[5] - 6;
    std::cout << ans << "\n";

    return 0;
}