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

using i64 = long long;

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

    auto solve = [&]() {
        int n;
        cin >> n;

        string s;
        cin >> s;

        string t = "spbsu";
        constexpr int inf = 1e9;
        vector<int> dp(6, inf), coeff{-1, -1, 0, 1, 1};
        dp[0] = 0;

        for (int i = 0; i < n; i++) {
            for (int j = 4; j >= 0; j--) {
                if (s[i] == t[j]) {
                    dp[j + 1] = min(dp[j + 1], dp[j] + coeff[j] * i);
                }
            }
        }

        int ans = dp[5] - 6;
        cout << ans << '\n';
    };
    
    solve();
    
    return 0;
}