アルゴリズムと数学 087

https://atcoder.jp/contests/math-and-algorithm/tasks/math_and_algorithm_bq

$\displaystyle \sum_{i=1}^N{\sum_{j=1}^N{ij}} = \sum_{i=1}^N{i}\sum_{j=1}^N{j} = (\sum_{i=1}^N{i})^2$
ですね。

// Simple Math Easy
#![allow(non_snake_case)]


//////////////////// 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()
}


//////////////////// process ////////////////////

fn f(N: u64) -> u64 {
    let D: u64 = 10u64.pow(9) + 7;
    let M = N * (N + 1) / 2 % D;
    M * M % D
}

fn main() {
    let N = read();
    println!("{}", f(N))
}