// 誤解法
#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;

i32 solve(const string& s) {
  regex pon("pon");
  auto tmp = s;
  while (true) {
    auto nxt = regex_replace(tmp, pon, "");
    if (nxt == tmp) break;
    tmp = nxt;
  }
  return (size(s) - size(tmp)) / 3;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  i32 n;
  cin>>n;
  string s;
  cin>>s;
  i32 ans = -1;
  rep2(i,1,n-1) {
    i32 l = solve(s.substr(0,i));
    i32 r = solve(s.substr(i));
    if (l && r) ans = max(ans, l+r-2);
  }
  cout<<ans<<endl;
  return 0;
}