Problem:
Score : 400 points

Problem Statement

Takahashi has N sticks that are distinguishable from each other. The length of the i-th stick is L_i.

He is going to form a triangle using three of these sticks. Let a, b, and c be the lengths of the three sticks used. Here, all of the following conditions must be satisfied:

a < b + c

b < c + a

c < a + b

How many different triangles can be formed? Two triangles are considered different when there is a stick used in only one of them.

Constraints

All values in input are integers.

3 \leq N \leq 2 \times 10^3

1 \leq L_i \leq 10^3

Input

Input is given from Standard Input in the following format:

N
L_1 L_2 ... L_N

Constraints

Print the number of different triangles that can be formed.

Sample Input 1

4
3 4 2 1

Sample Output 1

1

Only one triangle can be formed: the triangle formed by the first, second, and third sticks.

Sample Input 2

3
1 1000 1

Sample Output 2

0

No triangles can be formed.

Sample Input 3

7
218 786 704 233 645 728 389

Sample Output 3

23

Solution in Rust (1.15.1) (judge verdict: Time Limit Exceeded)

#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]

use std::io::*;
/// Example
/// ```
///
///  let n: i32 = read();
///
///  let mut xs = Vec::new();
///  for _ in 0..n {
///      xs.push(read::<i32>() + 1000);
///  }
/// ```
fn read<T: std::str::FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    ($arg:expr) => (println!("{:?}",$arg));
}


#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}


use std::cmp;
use std::collections::HashMap;


#[allow(dead_code)]
static INF_I64 :i64 = 9223372036854775807;
#[allow(dead_code)]
static INF_I32 : i32 = 2147483647;
#[allow(dead_code)]
static INF_USIZE : usize = 18446744073709551615;



fn main() {



    let n:usize = read();
    let mut l = vec![0_u32;n];
    for i in 0..n{
        l[i] = read();
    }

    let mut ans = 0;

    let mut l = l;
    l.sort();


    for i in 0..n{
        let a = l[i];
        for j in i+1..n{
            let ab = a + l[j];
            for k in j+1..n{
                if  ab > l[k]{
                    ans+=1;
                }
                else{
                    break;
                }
            }
        }
    }
    
    
    println!("{}",ans);


}