#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000000;
int main(){
  int n;
  cin >> n;
  string s;
  cin >> s;
  vector<vector<int>> dp(6, vector<int>(n + 1, INF));
  dp[0][0] = 0;
  for (int i = 0; i < n; i++){
    for (int j = 0; j <= 5; j++){
      dp[j][i + 1] = min(dp[j][i + 1], dp[j][i]);
    }
    if (s[i] == 's'){
      dp[1][i + 1] = min(dp[1][i + 1], dp[0][i] - i);
    }
    if (s[i] == 'p'){
      dp[2][i + 1] = min(dp[2][i + 1], dp[1][i] - i);
    }
    if (s[i] == 'b'){
      dp[3][i + 1] = min(dp[3][i + 1], dp[2][i]);
    }
    if (s[i] == 's'){
      dp[4][i + 1] = min(dp[4][i + 1], dp[3][i] + i);
    }
    if (s[i] == 'u'){
      dp[5][i + 1] = min(dp[5][i + 1], dp[4][i] + i);
    }
  }
  cout << dp[5][n] - 6 << endl;
}