import java.io.*;
import java.util.*;

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);

		int n = Integer.parseInt(in.readLine());
		String s = in.readLine();
		char[] msg = new char[] {'s', 'p', 'b', 's', 'u'};
		int[][] dp = new int[n][5];
		for (int i = 0; i < n; i++) {
			Arrays.fill(dp[i], Integer.MAX_VALUE);
			if (s.charAt(i) == msg[0]) {
				dp[i][0] = 0;
			}
			for (int j = 0; j < 5; j++) {
				if (j > 0 && s.charAt(i) == msg[j]) {
					if (i > 0) {
						dp[i][j] = Math.min(dp[i][j], dp[i - 1][j - 1]);
					}
				}
				int add = 1;
				if (j == 1 || j == 2) {
					add = 2;
				} else if (j == 4) {
					add = 0;
				}
				if (i > 0 && dp[i - 1][j] < Integer.MAX_VALUE) {
					dp[i][j] = Math.min(dp[i][j], dp[i - 1][j] + add);
				}
			}
		}
		out.println(dp[n - 1][4]);
		
		in.close();
		out.close();
	}
}