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

const ll N = 1e6 + 1;
const ll M = 1e9 + 7;

string s, pat = " spbsu";
vector<ll> a[N];

void doTest(ll testID) {
    ll n; cin >> n;
    cin >> s; s = ' ' + s;

    for (int i = 1; i <= n; i ++) a[s[i]].push_back(i);
    
    ll res = 1e18;
    for (int i = 0; i < a['b'].size(); i ++) {
        ll pos = i, tmp = 0, cur = a['b'][i], ok = 1; 
        
        for (int j = 4; j <= 5; j ++) {
            auto it = lower_bound(a[pat[j]].begin(), a[pat[j]].end(), cur);
            if (it == a[pat[j]].end()) {ok = 0; break;}
            ll nxt = *it; tmp += nxt - a['b'][i]; cur = nxt + 1; 
        }
        if (!ok) continue;

        cur = a['b'][i];
        for (int j = 2; j >= 1; j --) {
            ll l = 0, r = a[pat[j]].size() - 1, mid, res = -1;
            // cout << "Spec: " << j << ' ' << l << ' ' << r << ' ' << cur << endl;
            while (r >= l) {
                mid = (r + l) >> 1;
                if (a[pat[j]][mid] <= cur) l = (res = mid) + 1;
                else r = mid - 1;
            }
            if (res == -1) {ok = 0; break;}
            res = a[pat[j]][res];
            tmp += a['b'][i] - res; cur = res - 1;
        }
        if (!ok) continue;

        // cout << "Debug: " << i << " " << tmp << endl;
        res = min(res, tmp);
    }
    cout << res - 6;
}

signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0);

    int test = 1; 
    // cin >> test;
    for (int _ = 1; _ <= test; _ ++) doTest(test);
}