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

using ll = long long;
#define int ll
#define all(v) (v).begin(), (v).end()
#define pb push_back

void dbg_out() { cerr << endl; }
template <typename H, typename... T>
void dbg_out(H h, T... t) { cerr << ' ' << h; dbg_out(t...); }
#define dbg(...) { cerr << #__VA_ARGS__ << ':'; dbg_out(__VA_ARGS__); }

void solve() {
	int n; cin >> n;
	string b = "spbsu";
	string s; cin >> s;
	vector<vector<int>> occ(26);
	for(int i=0;i<n;i++) {
		occ[s[i] - 'a'].push_back(i);
	}

	int ans = 1e18;
	for(int i=0;i<n;i++) if(s[i] == 'b') {
		vector<int> l{i}, r{i};
		for(int d=1;d>=0;d--) {
			auto &o = occ[b[d] - 'a'];
			auto it = lower_bound(all(o), l.back());
			if(it == o.begin()) break;
			l.push_back(*prev(it));
		}
		for(int d=3;d<=4;d++) {
			auto &o = occ[b[d] - 'a'];
			auto it = upper_bound(all(o), r.back());
			if(it == o.end()) break;
			r.push_back(*it);
		}
		if(l.size() < 3 || r.size() < 3) continue;

		vector<int> p{l[2], l[1], i, r[1], r[2]};
		int cost = 0;
		cost += p[1] - p[0] - 1;
		cost += 2*(p[2] - p[1] - 1);
		cost += 2*(p[3] - p[2] - 1);
		cost += p[4] - p[3] - 1;
		ans = min(ans, cost);

	}

	cout << ans << '\n';
}

signed main() {
	ios::sync_with_stdio(false); cin.tie(0);
	solve();
}