#include <bits/stdc++.h>
#define SZ(x) (int) x.size() - 1
#define all(x) x.begin(), x.end()
#define ms(x, y) memset(x, y, sizeof x)
#define F(i, x, y) for (int i = (x); i <= (y); i++)
#define DF(i, x, y) for (int i = (x); i >= (y); i--)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
template <typename T> inline void chkmax(T& x, T y) { x = max(x, y); }
template <typename T> inline void chkmin(T& x, T y) { x = min(x, y); }
template <typename T> inline void read(T &x) {
	x = 0; int f = 1; char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
	for (; isdigit(c); c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
	x *= f;
}
// const int N = 1e5 + 10;
// int ans = 1e9;
// vector <int> tt[1010];
// void work(vector <int> v) {
// 	// reverse(all(v));
// 	int sum = 0;
// 	for (int i: v) sum += i - 1;
// 	F(i, 0, SZ(v))
// 		F(j, i + 1, SZ(v))
// 			sum += (v[i] < v[j]);
// 	for (int i: v) cout << i << " "; cout << " -> " << sum << endl;
// 	chkmin(ans, sum);
// }
// string ss = " spbsu";
int lst[1010], s1[100010], s2[100010], ans = 1e9;
signed main() {
	int n; string st; cin >> n >> st;
	st = ' ' + st;
	s1[0] = 1e9;
	F(i, 1, n) {
		lst[st[i]] = i;
		if (st[i] == 'p' && lst['s']) s1[i] = i - lst['s'] - 1;
		else s1[i] = s1[i - 1] + 2;
	}
	s2[n + 1] = 1e9;
	ms(lst, 0);
	DF(i, n, 1) {
		lst[st[i]] = i;
		if (st[i] == 's' && lst['u']) s2[i] = lst['u'] - i - 1;
		else s2[i] = s2[i + 1] + 2;
	}
	F(i, 1, n) {
		if (st[i] == 'b') {
			chkmin(ans, s1[i - 1] + s2[i + 1]);
			// cout << s1[i - 1] << " " << s2[i + 1] << endl;
		}
	}
	// ms(lst, -0x3f);
	// F(i, 1, n) {
	// 	// lst[st[i]] = i;
	// 	tt[st[i]].push_back(i);
	// 	if (tt['s'].size() > 1 && tt['p'].size() && tt['b'].size() && tt['u'].size())
	// 	work(vector <int> {i - tt['s'][1], i - tt['p'][0], i - tt['b'][0], i - tt['s'][0], i - tt['u'][0]});
	// }
	cout << ans;
	return 0;
}