use std::io::Read; fn get_word() -> String { let stdin = std::io::stdin(); let mut stdin=stdin.lock(); let mut u8b: [u8; 1] = [0]; loop { let mut buf: Vec = Vec::with_capacity(16); loop { let res = stdin.read(&mut u8b); if res.unwrap_or(0) == 0 || u8b[0] <= b' ' { break; } else { buf.push(u8b[0]); } } if buf.len() >= 1 { let ret = String::from_utf8(buf).unwrap(); return ret; } } } #[allow(dead_code)] fn get() -> T { get_word().parse().ok().unwrap() } fn main() { let n: usize = get(); let mut pr = vec![true; n + 1]; pr[0] = false; pr[1] = false; for i in 2..n + 1 { if !pr[i] { continue; } for j in 2..n / i + 1 { pr[i * j] = false; } } let mut tot = 0; for r in 2..n + 1 { if !pr[r] { continue; } if r * r - 2 <= n && pr[r * r - 2] { tot += if r == 2 { 1 } else { 2 }; } } println!("{}", tot); }