#include <bits/stdc++.h>
#include <variant>

#define rep2(i,k,n) for(i64 i=(i64)(k);i<(i64)(n);i++)
#define rep(i,n) rep2(i,0,n)
#define all(x) begin(x),end(x)
#ifdef ENV_LOCAL
#define dump if (1) cerr
#else
#define dump if (0) cerr
#endif

using namespace std;
using namespace std::string_literals;

using i32 = int32_t;
using i64 = int64_t;
using f64 = double;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;

//using namespace harudake;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  i32 n;
  cin>>n;
  string s;
  cin>>s;
  // dp[length][first]
  vector dp(n+1, vi32(n));
  string pon = "pon";
  rep2(len,1,n+1) {
    rep(i,n) {
      if (i + len > n) break;
      rep2(j,i,i+len-1) {
        i32 len_left = j-i+1;
        if (s[i+len-1] != pon[dp[len_left][i] % 3]) continue;
        i32 len_innder = (i+len-2) - (j+1) + 1;
        i32 inner = dp[len_innder][j+1];
        inner -= inner % 3;
        dp[len][i] = max(dp[len][i], dp[len_left][i] + inner + 1);
      }
      if (s[i+len-1] == pon[dp[len-1][i]%3]) {
        dp[len][i] = max(dp[len][i], dp[len-1][i]+1);
      }
      dp[len][i] = max(dp[len][i], dp[len-1][i]);
      if (len >= 2) dp[len][i] = max(dp[len][i], dp[len-1][i+1]);
      dump<<dp[len][i]<<" ";
    }
    dump<<endl;
  }
  i32 mx = -1;
  rep2(i,1,n-1) {
    i32 left = dp[i][0];
    i32 right = dp[n-i][i];
    left -= left % 3;
    right -= right % 3;
    if (left && right) {
      mx = max(mx, (left + right) / 3);
    }
  }
  if (mx >= 2) {
    cout<<(mx - 2)<<endl;
  } else {
    cout<<-1<<endl;
  }
  return 0;
}