#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using vi = vector<int>;
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define smx(a, b) a = max(a, b);
#define smn(a, b) a = min(a, b);
#define pb push_back
#define endl '\n'

const ll MOD = 1e9 + 7;
const ld EPS = 1e-9;

mt19937 rng(time(0));

int main() {
	cin.tie(0)->sync_with_stdio(0);
    int n; cin >> n;
    string s; cin >> s;
    

    int dp[100010][6] = {};
    for(int i = 0; i < 100010; i++) {
        for(int j = 0; j < 6; j++) dp[i][j] = 1e9;
    }
    string ref = "spbsu";
    dp[0][0] = 0;
    int w[5] = {-1,-1,0,1,1};
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < 6; j++) {
            dp[i + 1][j] = min(dp[i+1][j], dp[i][j]);
        }
        for(int j = 0; j < 5; j++) {
            if(s[i] == ref[j]) {
                dp[i+1][j+1] = min(dp[i+1][j+1], dp[i][j] + i * w[j]);
            }
        }
    }

    cout << dp[n][5] - 6 << endl;
}