#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


constexpr int INF = 1001001001;
constexpr int K = 5;
constexpr const char *T = "spbsu";

int N;
char S[100'010];

int main() {
  for (; ~scanf("%d", &N); ) {
    scanf("%s", S);
    
    int dp[K + 1] = {};
    fill(dp, dp + (K + 1), INF);
    dp[0] = 0;
    for (int i = 0; i < N; ++i) {
      for (int k = K; --k >= 0; ) if (S[i] == T[k]) {
        chmin(dp[k + 1], dp[k] + ((k < K/2) ? -1 : (k == K/2) ? 0 : +1) * i);
      }
    }
    int ans = dp[K];
    for (int k = 0; k < K; ++k) {
      ans -= abs(k - K/2);
    }
    printf("%d\n", ans);
  }
  return 0;
}