#include <bits/stdc++.h>
#ifdef SHARAELONG
#include "../../cpp-header/debug.hpp"
#endif
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;

void solve() {
    int n;
    string s;
    cin >> n >> s;

    vector<vector<int>> freq(26);
    for (int i=0; i<n; ++i) {
        freq[s[i]-'a'].push_back(i);
    }

    vector<int> fr(n, 1e8);
    vector<int> bck(n, 1e8);
    for (int i=0; i<n; ++i) {
        if (s[i] == 'p') {
            auto it = lower_bound(freq['s'-'a'].begin(), freq['s'-'a'].end(), i);
            if (it == freq['s'-'a'].begin()) continue;
            int idx = *prev(it);
            fr[i] = i-idx-1;
        } else if (s[i] == 'b') {
            auto it = lower_bound(freq['p'-'a'].begin(), freq['p'-'a'].end(), i);
            if (it == freq['p'-'a'].begin()) continue;
            int idx = *prev(it);
            fr[i] = 2*(i-idx-1) + fr[idx];
        }
    }

    for (int i=n-1; i>=0; --i) {
        if (s[i] == 's') {
            auto it = lower_bound(freq['u'-'a'].begin(), freq['u'-'a'].end(), i);
            if (it == freq['u'-'a'].end()) continue;
            int idx = *it;
            bck[i] = idx-i-1;
        } else if (s[i] == 'b') {
            auto it = lower_bound(freq['s'-'a'].begin(), freq['s'-'a'].end(), i);
            if (it == freq['s'-'a'].end()) continue;
            int idx = *it;
            bck[i] = 2*(idx-i-1) + bck[idx];
        }
    }

    int ans = 1e9;
    for (int i=0; i<n; ++i) {
        if (s[i] == 'b') ans = min(ans, fr[i] + bck[i]);
    }
    cout << ans;
}

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

    solve();
}