// ---------- begin chmin, chmax ----------
pub trait ChangeMinMax {
    fn chmin(&mut self, x: Self) -> bool;
    fn chmax(&mut self, x: Self) -> bool;
}

impl<T: PartialOrd> ChangeMinMax for T {
    fn chmin(&mut self, x: Self) -> bool {
        *self > x && {
            *self = x;
            true
        }
    }
    fn chmax(&mut self, x: Self) -> bool {
        *self < x && {
            *self = x;
            true
        }
    }
}
// ---------- end chmin, chmax ----------
// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------

fn run() {
    input! {
        n: usize,
        s: bytes,
    }
    let mut dp = vec![vec![0; n]; n];
    for l in (0..(n - 1)).rev() {
        for r in (l + 1)..n {
            let mut val = 0;
            for j in l..r {
                val.chmax(dp[l][j] + dp[j + 1][r]);
            }
            if s[l] == b'p' && s[r] == b'n' {
                for j in (l + 1)..r {
                    if s[j] == b'o' {
                        val.chmax(1 + dp[l + 1][j - 1] + dp[j + 1][r - 1]);
                    }
                }
            }
            dp[l][r] = val;
        }
    }
    let cost = dp;
    let mut dp = vec![-1; n + 2];
    dp[0] = 0;
    for c in "ponpon".bytes() {
        let mut next = vec![-1; n + 2];
        for (i, dp) in dp.iter().enumerate() {
            if *dp < 0 {
                continue;
            }
            for j in i..n {
                if s[j] == c {
                    let mut val = *dp;
                    if i < j {
                        val += cost[i][j - 1];
                    }
                    next[j + 1].chmax(val);
                }
            }
        }
        dp = next;
    }
    println!("{}", dp.iter().max().unwrap());
}

fn main() {
    run();
}