Problem:
Score : 200 points

Problem Statement

It's now the season of TAKOYAKI FESTIVAL!

This year, N takoyaki (a ball-shaped food with a piece of octopus inside) will be served. The deliciousness of the i-th takoyaki is d_i.

As is commonly known, when you eat two takoyaki of deliciousness x and y together, you restore x \times y health points.

There are \frac{N \times (N - 1)}{2} ways to choose two from the N takoyaki served in the festival. For each of these choices, find the health points restored from eating the two takoyaki, then compute the sum of these \frac{N \times (N - 1)}{2} values.

Constraints

All values in input are integers.

2 \leq N \leq 50

0 \leq d_i \leq 100

Input

Input is given from Standard Input in the following format:

N
d_1 d_2 ... d_N

Output

Print the sum of the health points restored from eating two takoyaki over all possible choices of two takoyaki from the N takoyaki served.

Sample Input 1

3
3 1 2

Sample Output 1

11

There are three possible choices:

Eat the first and second takoyaki. You will restore 3 health points.

Eat the second and third takoyaki. You will restore 2 health points.

Eat the first and third takoyaki. You will restore 6 health points.

The sum of these values is 11.

Sample Input 2

7
5 0 7 8 3 3 2

Sample Output 2

312

Solution in Rust (1.15.1) (judge verdict: Accepted)

// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8

macro_rules! input {
    (source = $s:expr; $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        #[allow(unused_mut)]
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
#[allow(unused_imports)]
use std::cmp::*;
trait Each: Iterator {
    fn each<F>(self, mut f: F)
    where
        Self: Sized,
        F: FnMut(Self::Item),
    {
        self.fold((), move |(), item| f(item));
    }
}
impl<T: Iterator> Each for T {}

const SEPARATOR: &'static str = " ";

fn solve(source: &str) -> String {
    let mut result: Vec<String> = vec![];
    macro_rules! p {
        ([$x: expr]) => {
            $x.iter().each(|&x| p!(x))
        };
        ([$x: expr]; $s:expr)=>{
            p!($x.iter().map(|x|x.to_string()).collect::<Vec<_>>().join($s));
        };
        ($($x: expr),* $(,)*) => {
            $(result.push($x.to_string()));*
        };
    }
    //---------------------------------------------------------------//

    input! {
        source = source;
        n:usize,
        d:[usize;n]
    }
    let mut ans = 0;
    for i in 0..n {
        for j in i+1..n {
            ans += d[i] * d[j];
        }
    }
    p!(ans);
    //---------------------------------------------------------------//
    return result.join(SEPARATOR);
}
fn main() {
    let mut s;
    let source = {
        use std::io::Read;
        s = String::new();
        std::io::stdin().read_to_string(&mut s).unwrap();
        s.as_str()
    };
    println!("{}", solve(source));
}
#[test]
fn testcase() {
    assert_eq!(
        solve(
            r"3
3 1 2
"
        ),
        r"11"
    );
    assert_eq!(
        solve(
            r"7
5 0 7 8 3 3 2
"
        ),
        r"312"
    );
}