#include <bits/stdc++.h>
using namespace std;
int main() {
#define int long long
	int N; string S;
	cin >> N >> S;
	// find subsequence spbsu

	vector<vector<int>> DP(6,vector<int>(N, 0));

	int ans = N*N;

	string target = "spbsu";

	for(int i = 1; i < 6; i++) {
		int ppv = (i == 1) ? 0 : N*N;
		int pv = N*N;
		for(int j = 0; j < N; j++) {
			DP[i][j] = pv+min(i,(5-i)); //skip.
			if(S[j] == target[i-1]) DP[i][j] = min(DP[i][j], ppv);
			pv = DP[i][j];
			ppv = DP[i-1][j];
			//cout << i << " " << " " << j << " " << DP[i][j] << endl;
			if(i == 5) ans = min(ans, pv); 
		}
	}


	cout << ans << endl;


}