#include "bits/stdc++.h"
#define inf 10000000

using namespace std;

int main(void){
    ios_base::sync_with_stdio(false), cin.tie(nullptr);
    
    int n;
    cin >> n;

    vector<int> pos_s = {-1, inf}, pos_p = {-1, inf}, pos_u = {-1, inf}, pos_b;
    for(int i=0; i<n; i++) {
        char c;
        cin >> c;

        if(c == 's') pos_s.push_back(i);
        else if(c == 'p') pos_p.push_back(i);
        else if(c == 'u') pos_u.push_back(i);
        else if(c == 'b') pos_b.push_back(i);
    }
    sort(pos_s.begin(), pos_s.end());
    sort(pos_p.begin(), pos_p.end());
    sort(pos_u.begin(), pos_u.end());
    sort(pos_b.begin(), pos_b.end());

    auto resp = inf;
    for(auto pb: pos_b) {       
        auto it_pp = lower_bound(pos_p.begin(), pos_p.end(), pb); it_pp--;
        auto pp = *it_pp;
        if(pp == -1) continue;
        auto it_ps = lower_bound(pos_s.begin(), pos_s.end(), pp); it_ps--;
        auto ps = *it_ps;
        if(ps == -1) continue;

        auto ps2 = *lower_bound(pos_s.begin(), pos_s.end(), pb);
        if(ps2 == inf) continue;
        auto pu = *lower_bound(pos_u.begin(), pos_u.end(), ps2);
        if(pu == inf) continue;

        int r = abs(pp-pb) + abs(ps-pb) + abs(ps2-pb) + abs(pu-pb) - 6;
        resp = min(resp, r);
        // cout << " | " << pp << " " << ps << " " << ps2 << " " << pu << endl;
        // cout << *ps2 << " " << *pu << endl;
    }

    cout << resp << endl;

    return 0;
}