https://atcoder.jp/contests/math-and-algorithm/tasks/math_and_algorithm_br
しらみつぶしで十分速いです。
// How Many Ways? #![allow(non_snake_case)] use std::cmp::min; //////////////////// library //////////////////// fn read<T: std::str::FromStr>() -> T { let mut line = String::new(); std::io::stdin().read_line(&mut line).ok(); line.trim().parse().ok().unwrap() } fn read_vec<T: std::str::FromStr>() -> Vec<T> { read::<String>().split_whitespace() .map(|e| e.parse().ok().unwrap()).collect() } //////////////////// process //////////////////// fn read_input() -> (i32, i32) { let v = read_vec(); let N = v[0]; let X = v[1]; (N, X) } fn f(N: i32, X: i32) -> u32 { let mut counter = 0; let max_a = min(N, X/3-1); for a in 1..max_a+1 { let max_b = min(N, (X-a-1)/2); for b in a+1..max_b+1 { let c = X - a - b; if 1 <= c && c <= N { counter += 1 } } } counter } fn main() { let (N, X) = read_input(); println!("{}", f(N, X)) }