#[allow(unused_imports)] use std::cmp::*; #[allow(unused_imports)] use std::collections::*; // https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8 macro_rules! input { ($($r:tt)*) => { let stdin = std::io::stdin(); let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock())); let mut next = move || -> String{ bytes.by_ref().map(|r|r.unwrap() as char) .skip_while(|c|c.is_whitespace()) .take_while(|c|!c.is_whitespace()) .collect() }; input_inner!{next, $($r)*} }; } macro_rules! input_inner { ($next:expr) => {}; ($next:expr,) => {}; ($next:expr, $var:ident : $t:tt $($r:tt)*) => { let $var = read_value!($next, $t); input_inner!{$next $($r)*} }; } macro_rules! read_value { ($next:expr, ( $($t:tt),* )) => { ($(read_value!($next, $t)),*) }; ($next:expr, [ $t:tt ; $len:expr ]) => { (0..$len).map(|_| read_value!($next, $t)).collect::>() }; ($next:expr, chars) => { read_value!($next, String).chars().collect::>() }; ($next:expr, usize1) => (read_value!($next, usize) - 1); ($next:expr, [ $t:tt ]) => {{ let len = read_value!($next, usize); read_value!($next, [$t; len]) }}; ($next:expr, $t:ty) => ($next().parse::<$t>().expect("Parse error")); } trait Change { fn chmax(&mut self, x: Self); fn chmin(&mut self, x: Self); } impl Change for T { fn chmax(&mut self, x: T) { if *self < x { *self = x; } } fn chmin(&mut self, x: T) { if *self > x { *self = x; } } } fn main() { input! { n: usize, s: chars, } let pon = ['p', 'o', 'n']; const INF: i32 = 1 << 28; let mut dp = vec![vec![[-INF; 3]; n + 1]; n]; for i in 0..n { for j in i..n + 1 { dp[i][j][0] = 0; } } for t in 3..n + 1 { for i in 0..n - t + 1 { let j = i + t; for b in 0..3 { let mut ma = -INF; if b <= 1 { for k in 0..3 { if s[i..i + k] == pon[..k] && s[j + k - 3..j] == pon[k..] { ma = max(ma, 1 + dp[i + k][j + k - 3][0]); } } } for c in 0..b + 1 { for k in i + 1..j { ma = max(ma, dp[i][k][c] + dp[k][j][b - c]); } } if s[i] == 'p' && s[j - 1] == 'n' && b <= 1 { for k in i + 1..j - 1 { if s[k] == 'o' && dp[i + 1][k][0] >= 0 && dp[k + 1][j - 1][0] >= 0 { ma = max(ma, 1 + dp[i + 1][k][0] + dp[k + 1][j - 1][0]); } } } ma = max(ma, dp[i][j - 1][b]); ma = max(ma, dp[i + 1][j][b]); dp[i][j][b] = ma; } } } println!("{}", max(-1, dp[0][n][2] - 2)); }