#include <bits/stdc++.h>
using namespace std;
const int INF = 100000;
int main(){
  int N;
  cin >> N;
  string S;
  cin >> S;
  string T = "ponpon";
  vector<vector<int>> dp(N + 1, vector<int>(7, -INF));
  dp[0][0] = 0;
  for (int i = 0; i < N; i++){
    for (int j = 0; j <= 6; j++){
      if (dp[i][j] != -INF){
        if (S[i] == T[dp[i][j] % 3]){
          dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + 1);
        } else {
          dp[i + 1][j] = max(dp[i + 1][j], dp[i][j]);
        }
        if (j < 6){
          if (S[i] == T[j]){
            dp[i + 1][j + 1] = max(dp[i + 1][j + 1], dp[i][j] / 3 * 3);
          }
        }
      }
    }
  }
  if (dp[N][6] == -INF){
    cout << -1 << endl;
  } else {
    cout << dp[N][6] / 3 << endl;
  }
}