#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  string S;
  cin >> S;
  vector<vector<int>> dp(N + 1, vector<int>(N + 1, 0));
  for (int i = N; i >= 0; i--){
    for (int j = i + 1; j <= N; j++){
      if (S[i] == 'p' && S[j - 1] == 'n'){
        for (int k = i + 1; k < j - 1; k++){
          if (S[k] == 'o'){
            dp[i][j] = max(dp[i][j], dp[i + 1][k] + dp[k + 1][j - 1] + 1);
          }
        }
        dp[i][j] = max(dp[i][j], dp[i + 1][j]);
        dp[i][j] = max(dp[i][j], dp[i][j - 1]);
      }
    }
  }
  S += '$';
  N++;
  string T = "ponpon$";
  vector<vector<int>> dp2(N + 1, vector<int>(8, -1));
  dp2[0][0] = 0;
  for (int i = 0; i < N; i++){
    for (int j = i; j < N; j++){
      for (int k = 0; k < 7; k++){
        if (dp2[i][k] != -1 && S[j] == T[k]){
          dp2[j + 1][k + 1] = max(dp2[j + 1][k + 1], dp2[i][k] + dp[i][j]);
        }
      }
    }
  }
  if (dp2[N][7] == -1){
    cout << -1 << endl;
  } else {
    assert(false);
    cout << dp2[N][7] << endl;
  }
}