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

using ll = long long;

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

    int n;
    cin >> n;
    string s;
    cin >> s;
    vector<vector<int>> pos(26);
    for (int i = 0; i < n; ++i) {
        pos[s[i] - 'a'].push_back(i);
    }
    string t = "spbsu";
    int ans = 1e9;
    for (int i = 0; i < n; ++i) {
        if (s[i] != 'b') continue;
        int sum = 0;
        int cur = i;
        for (auto x : {'p', 's'}) {
            int c = x - 'a';
            if (pos[c].empty() || pos[c][0] >= cur) {
                sum = 1e9;
                break;
            }
            cur = *--lower_bound(pos[c].begin(), pos[c].end(), cur);
            sum += i - cur;
        }
        cur = i;
        for (auto x : {'s', 'u'}) {
            int c = x - 'a';
            if (pos[c].empty() || pos[c].back() <= cur) {
                sum = 1e9;
                break;
            }
            cur = *upper_bound(pos[c].begin(), pos[c].end(), cur);
            sum += cur - i;
        }
        sum -= 6;
        ans = min(ans, sum);
    }
    cout << ans;
}