#include <bits/stdc++.h>

using i64 = long long;

constexpr int inf = 1E9;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int n;
    std::cin >> n;
    
    std::string s;
    std::cin >> s;
    
    const std::string t = "spbsu";
    std::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] = std::min(dp[j + 1], dp[j] + (j < 2 ? -1 : j > 2 ? 1 : 0) * i);
            }
        }
    }
    
    int ans = dp[5] - 6;
    std::cout << ans << "\n";
    
    return 0;
}