#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n; string s; cin >> n >> s;
  vector dp(n, vector(n, 0));
  for (int len = 3; len <= n; ++len) {
    for (int i = 0; i + len <= n; ++i) {
      int j = i + len - 1;
      FOR(k, i, j) chmax(dp[i][j], dp[i][k] + dp[k + 1][j]);
      if (s[i] == 'p' && s[j] == 'n') {
        FOR(k, i + 1, j) {
          if (s[k] == 'o'){
            chmax(dp[i][j], (i + 1 == k ? 0 : dp[i + 1][k - 1]) + (k + 1 == j ? 0 : dp[k + 1][j - 1]) + 1);
          }
        }
      }
    }
  }
  int ans = -1;
  FOR(i, 3, n) {
    if (dp[0][i - 1] > 0 && dp[i][n - 1] > 0) chmax(ans, dp[0][i - 1] + dp[i][n - 1] - 2);
  }
  cout << ans << '\n';
  return 0;
}