#include <bits/stdc++.h>

using namespace std;

#define iter(v) v.begin(), v.end()
#define SZ(v) int(v.size())
#define pb emplace_back
#define ff first
#define ss second

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#ifdef zisk
void debug(){cout << endl;}
template<class T, class ... U>
void debug(T a, U ... b){ cout << a << " ", debug(b...); }
template<class T> void pary(T l, T r){
	while(l != r) cout << *l << " ", l++;
	cout << endl;
}
#else
#define debug(...) void()
#define pary(...) void()
#endif

template<class A, class B>
ostream& operator<<(ostream& o, pair<A, B> p){
	return o << '(' << p.ff << ',' << p.ss << ')';
}

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

	int n;
	cin >> n;
	string s;
	cin >> s;

	const int MAX = 1e9;
	string code = " spbsu";

	auto solve = [&](vector<vector<int>> &ans){
		//debug("solve");
		vector<int> dp(6, MAX);
		dp[0] = 0;
		for(int i = 0; i < n; i++){
			int cur = MAX;
			for(int j = 5; j >= 0; j--){
				if(code[j] != s[i]){
					dp[j] += j;
				}
				else{
					dp[j] = dp[j - 1];
					cur = min(cur, dp[j]);
				}
			}
			ans[i] = dp;
		}
	};

	vector<vector<int>> ans1(n);
	vector<vector<int>> ans2(n);
	solve(ans1);
	reverse(iter(s));
	reverse(code.begin() + 1, code.end());
	solve(ans2);
	//debug("ok");

	int ans = MAX;
	for(int i = 0; i < n; i++){
		for(int j = 1; j <= 5; j++){
			ans = min(ans, ans1[i][j] + ans2[n - 1 - i][6 - j]);
		}
	}
	cout << ans << "\n";


}