Problem:
Score : 600 points

Problem Statement

We have an integer sequence A of length N, where A_1 = X, A_{i+1} = A_i + D (1 \leq i < N ) holds.

Takahashi will take some (possibly all or none) of the elements in this sequence, and Aoki will take all of the others.

Let S and T be the sum of the numbers taken by Takahashi and Aoki, respectively. How many possible values of S - T are there?

Constraints

-10^8 \leq X, D \leq 10^8

1 \leq N \leq 2 \times 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N X D

Output

Print the number of possible values of S - T.

Sample Input 1

3 4 2

Sample Output 1

8

A is (4, 6, 8).

There are eight ways for (Takahashi, Aoki) to take the elements: ((), (4, 6, 8)), ((4), (6, 8)), ((6), (4, 8)), ((8), (4, 6))), ((4, 6), (8))), ((4, 8), (6))), ((6, 8), (4))), and ((4, 6, 8), ()).

The values of S - T in these ways are -18, -10, -6, -2, 2, 6, 10, and 18, respectively, so there are eight possible values of S - T.

Sample Input 2

2 3 -3

Sample Output 2

2

A is (3, 0). There are two possible values of S - T: -3 and 3.

Sample Input 3

100 14 20

Sample Output 3

49805

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

macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String {
            bytes
                .by_ref()
                .map(|r| r.unwrap() as char)
                .skip_while(|c| c.is_whitespace())
                .take_while(|c| !c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

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

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

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

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

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

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

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

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

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_imports)]
use std::cmp::{min, max};
use std::collections::HashMap;

fn main() {
    input!{
        n: usize,
        x: i64,
        d: i64
    }
    if d == 0 {
        if x == 0 {
            println!("1");
        } else {
            println!("{}", n + 1);
        }
        return;
    }
    let g = gcd(x.abs() as usize, d.abs() as usize);
    let x = x / g as i64 * if d < 0 {-1} else {1};
    let d = d.abs() / g as i64;
    let mut map = HashMap::new();
    for i in 0..n as i64 + 1 {
        let m = (i * x % d + d) % d;
        let v = map.entry(m).or_insert(vec![]);
        v.push(((i * x - m) / d + i * (i - 1) / 2, 1));
        v.push(((i * x - m) / d + i * (2 * n as i64 - i - 1) / 2, -1));
    }
    let mut ans: usize = 0;
    for v in map.values() {
        let mut w = v.clone();
        w.sort_by(|x, y| if x.0 == y.0 {y.1.cmp(&x.1)} else {x.0.cmp(&y.0)});
        let mut s = 0;
        let mut c = 0;
        for &(p, f) in &w {
            if c == 0 {
                s = p;
            }
            c += f;
            if c == 0 {
                ans += (p + 1 - s) as usize;
            }
        }
    }
    println!("{}", ans);
}

fn gcd(n: usize, m: usize) -> usize {
    let mut l = min(n, m);
    let mut r = max(n, m);
    while l > 0 {
        let t = l;
        l = r % l;
        r = t;
    }
    r
}