#include <bits/stdc++.h>
using namespace std;

int dp[505][505];

int main() {
    int N; cin >> N;
    string S; cin >> S;
    for(int len = 1; len <= N; len++) {
        for(int i = 1; i + len - 1 <= N; i++) {
            int j = i + len - 1;
            dp[i][j] = max(dp[i][j - 1], dp[i + 1][j]);
            if(S[i - 1] == 'p' && S[j - 1] == 'n') {
                for(int k = i + 1; k < j; k++)
                    if(S[k - 1] == 'o')
                        dp[i][j] = max(dp[i][j], 1 + dp[i + 1][k - 1] + dp[k + 1][j - 1]);
            }
            for(int k = i; k <= j; k++) {
                dp[i][j] = max(dp[i][j], dp[i][k] + dp[k + 1][j]);
            }
        }
    }
    int ans = -1;
    for(int i = 1; i <= N; i++) {
        if(dp[1][i] >= 1 && dp[i + 1][N] >= 1)
            ans = max(ans, dp[1][i] + dp[i + 1][N] - 2);
    }
    cout << ans << "\n";
}