#include <bits/stdc++.h>
using namespace std;
int A[155757];
int ne[5][101010];
int pv[5][101010];
signed main() {
	ios::sync_with_stdio(false), cin.tie(0);
	int N;
	string s;
	cin >> N >> s;
	int i;
	for (i = 0; i < N; i++) {
		if (s[i] == 'p') A[i + 1] = 1;
		else if (s[i] == 'b') A[i + 1] = 2;
		else if (s[i] == 'u') A[i + 1] = 3;
		else if (s[i] != 's') A[i + 1] = 4;
	}
	int j;
	for (j = 0; j < 5; j++) ne[j][N + 1] = N + 1;
	for (j = 0; j < 5; j++) pv[j][0] = 0;
	for (i = N; i > 0; i--) {
		for (j = 0; j < 4; j++) ne[j][i] = ne[j][i + 1];
		ne[A[i]][i] = i;
	}
	for (i = 1; i <= N; i++) {
		for (j = 0; j < 4; j++) pv[j][i] = pv[j][i - 1];
		pv[A[i]][i] = i;
	}
	int mn = 1e9;
	for (i = 0; i < N; i++) {
		if (A[i] != 2) continue;
		int a, b, c, d, e;
		c = i;
		b = pv[1][c];
		a = pv[0][b];
		d = ne[0][c];
		e = ne[3][d];
		int sum = 0;
		if (e <= N && a >= 1) {
			sum += (d - c - 1) * 2;
			sum += (c - b - 1) * 2;
			sum += (e - d - 1);
			sum += (b - a - 1);
			mn = min(mn, sum);
		}
	}
	cout << mn;
}