#include <bits/stdc++.h>

using namespace std;

#define all(V) V.begin (), V.end ()

using LL = long long;

LL INF = 1e8;

vector <int> adj[26];

int previous (int c, int pos, int boshbe) {
    auto x = upper_bound (all (adj[c]), pos) - adj[c].begin () - 1;
    if (x == -1) return INF;
    else return boshbe - adj[c][x];
}

int next (int c, int pos, int boshbe) {
    // cout<<c<<' '<<pos<<' '<<boshbe<<'\n';
    // for(auto u: adj[c]) cout<<u<<' '; cout<<'\n';
    auto x = lower_bound (all (adj[c]), pos) - adj[c].begin ();
    if (x == adj[c].size ()) return INF;
    else return adj[c][x] - boshbe;
}

void solve (int tc) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    for (int i = 0; i < n; i++) {
        adj[s[i] - 'a'].push_back (i);
    }
    int ans = INF;  
    string spb = "spbsu";
    // cout<<next('u' - 'a', 9, 4)<<'\n';
    for (int i = 2; i < n - 2; i++) {
        if(s[i] == 'b'){

            int tmp = 0, last = i - 1;
            // cout<<i<<"--->";
            for(int j = 1; j >= 0; j--){
                // cout<<spb[j]<<" "<<previous(spb[j] - 'a', last, i - 2 + j)<<' ';
                int cur = previous(spb[j] - 'a', last, i - 2 + j);
                // cout<<cur<<' ';
                last = i - 2 + j - cur - 1;
                tmp += cur;
            }
            last = i + 1; //cout<<i + 3 - 2<<'\n';
            for(int j = 3; j < 5; j++){
                // cout<<last<<' ';
                // cout<<spb[j]<<" "<<next(spb[j] - 'a', last, i + j - 2)<<' ';
                int cur = next(spb[j] - 'a', last, i + j - 2);
                // cout<<cur<<' ';
                last = i - 2 + j + cur + 1;
                tmp += cur;
            }
            // cout<<'\n';
            ans = min(ans, tmp);
        }
    }
    cout<<ans<<'\n';
}

int main () {
    cin.tie (nullptr) -> ios_base :: sync_with_stdio (false);
    int tests=1;
    //cin >> tests;
    for (int tc = 1; tc <= tests; tc++) {
        solve (tc);
    }
    return 0;
}