#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
//const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) {
  if(b < 0)b *= -1;
  return a > b && (a = b, true);
}

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}


int main() {
  int N;
  cin >> N;
  string S;
  cin >> S;
  auto dp = make_v< int >(N, N);
  fill_v(dp, -1);
  auto rec = MFP([&](auto rec, int l, int r) -> int {
    if(l + 1 >= r) {
      return 0;
    }
    if(~dp[l][r]) {
      return dp[l][r];
    }
    int ret = max(rec(l + 1, r), rec(l, r - 1));
    if(S[l] == 'p' and S[l + 1] == 'o' and S[l + 2] == 'n') {
      chmax(ret, rec(l + 3, r) + 1);
    }
    if(S[r - 2] == 'p' and S[r - 1] == 'o' and S[r] == 'n') {
      chmax(ret, rec(l, r - 3) + 1);
    }
    if(S[l] == 'p' and S[r] == 'n') {
      for(int m = l + 1; m < r; m++) {
        if(S[m] == 'o') {
          chmax(ret, rec(l + 1, m - 1) + rec(m + 1, r - 1) + 1);
        }
      }
    }
    return dp[l][r] = ret;
  });

  const string temp = "ponpon";
  auto dp2 = make_v< int >(N + 1, 7);
  fill_v(dp2, -inf);
  dp2[0][0] = 0;
  for(int i = 0; i < N; i++) {
    for(int j = 0; j < 6; j++) {
      if(temp[j] == S[i]) {
        chmax(dp2[i + 1][j + 1], dp2[i][j]);
      }
    }
    for(int j = 0; j <= 6; j++) {
      for(int k = i + 1; k <= N; k++) {
        chmax(dp2[k][j], dp2[i][j] + rec(i, k - 1));
      }
    }
  }
  cout << max(dp2[N][6], -1) << "\n";
}