#include<bits/stdc++.h>

using namespace std;

#define int long long
constexpr int inf = 1E9;


int32_t main() {
    int n;
    string s;
    cin >> n >> s;
    const string t = "spbsu";
    array<int, 6> dp;
    dp.fill(inf), dp[0] = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 4; j >= 0; j--)
            if (t[j] == s[i])
                dp[j + 1] = min(dp[j + 1], dp[j] + (j < 2 ? -1 : j > 2 ? 1 : 0) * i);
    }
    cout << dp[5] - 6 << "\n";
    return 0;
}