use std::cmp::*; // 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; } } } type Coord = i128; // the type of coordinates fn half((x, y, _): (Coord, Coord, i32)) -> i32 { assert_ne!((x, y), (0, 0)); if y >= 0 { if x > 0 || y > 0 { 1 } else { 2 } } else { 2 } } // arg sort // Verified by: https://yukicoder.me/submissions/706856 fn arg_sort(xy: &mut [(Coord, Coord, i32)]) { xy.sort_unstable_by(|&a, &b| { half(a).cmp(&half(b)).then_with( || 0.cmp(&(a.0 * b.1 - a.1 * b.0)) .then_with(|| a.2.cmp(&b.2))) }); } fn main() { input! { n: usize, a: [(i128, i128); n], } if n == 1 { println!("0"); return; } let mut ans = 0; for i in 0..n { let (xi, yi) = a[i]; let mut b = vec![]; for j in 0..n { if i != j { b.push((a[j].0 - xi, a[j].1 - yi, j as i32)); } } arg_sort(&mut b); for i in 0..b.len() - 1 { let a = b[i]; let b = b[i + 1]; if half(a).cmp(&half(b)).then_with( || 0.cmp(&(a.0 * b.1 - a.1 * b.0))) == Ordering::Equal { ans += 1; break; } } } println!("{}", ans); }