#include <bits/stdc++.h>

constexpr int N = 5;
constexpr int INF = 1e9;
using dp_t = std::array<int, N + 1>;

int main() {
    int n;
    std::cin >> n;

    std::string s;
    std::cin >> s;

    dp_t dp;
    dp.fill(INF);
    dp[0] = 0;

    for (int i = 0; i < n; ++i) {
        dp_t ndp;
        for (int j = 0; j <= N; ++j) {
            int d = std::min(j, N - j);
            ndp[j] = dp[j] + d;
        }

        auto process = [&](int x) {
            ndp[x] = std::min(ndp[x], dp[x - 1]);
        };
        if (s[i] == 's') {
            process(1);
            process(4);
        } else if (s[i] == 'p') {
            process(2);
        } else if (s[i] == 'b') {
            process(3);
        } else if (s[i] == 'u') {
            process(5);
        }

        dp = ndp;
    }

    std::cout << dp[N] << '\n';
}