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

#define sz(x) (int)x.size()

string s, t = "spbsu";
vector<int> pos[26];

int main() {
    int n;
    cin >> n;
    cin >> s;
    long long ans = 1e15;
    for (int i = 0; i < 26; ++i)
        pos[i].push_back(-1);
    for (int i = 0; i < sz(s); ++i) {
        pos[s[i] - 'a'].push_back(i);
    }
    for (int i = 0; i < 26; ++i)
        pos[i].push_back(sz(s) + 200);
    for (int i = s.size() - 1; i >= 0; --i) {
        for (int j = 0; j < sz(t); ++j) {
            long long cnt = 0, f = 0;
            for (int k = j, p = i - 1, now = i; k < sz(t); ++k) {
                p = *upper_bound(pos[t[k] - 'a'].begin(), pos[t[k] - 'a'].end(), p);
                if (p == -1 || p == s.size() + 200) f = 1;
                cnt += abs(p - now);
                now++;
            }
            for (int k = j - 1, p = i + 1, now = i - 1; k >= 0; --k) {
                p = *prev(lower_bound(pos[t[k] - 'a'].begin(), pos[t[k] - 'a'].end(), p));
                if (p == -1 || p == s.size() + 200) f = 1;
                cnt += abs(p - now);
                now--;
            }
            if (f) continue;
            ans = min(ans, cnt);
        }
    }
    cout << ans << '\n';
}