Problem:
Score : 200 points

Problem Statement

In some village, there are 999 towers that are 1,(1+2),(1+2+3),...,(1+2+3+...+999) meters high from west to east, at intervals of 1 meter.

It had been snowing for a while before it finally stopped. For some two adjacent towers located 1 meter apart, we measured the lengths of the parts of those towers that are not covered with snow, and the results are a meters for the west tower, and b meters for the east tower.

Assuming that the depth of snow cover and the altitude are the same everywhere in the village, find the amount of the snow cover.

Assume also that the depth of the snow cover is always at least 1 meter.

Constraints

1 \leq a < b < 499500(=1+2+3+...+999)

All values in input are integers.

There is no input that contradicts the assumption.

Input

Input is given from Standard Input in the following format:

a b

Output

If the depth of the snow cover is x meters, print x as an integer.

Sample Input 1

8 13

Sample Output 1

2

The heights of the two towers are 10 meters and 15 meters, respectively.
Thus, we can see that the depth of the snow cover is 2 meters.

Sample Input 2

54 65

Sample Output 2

1

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

fn solve<R: BufRead, W: Write>(_reader: R, _writer: &mut W) {
  let mut _scanner = Scanner::new(_reader);

  #[allow(unused_macros)]
  macro_rules! scan {
    ($t:ty) => {
      _scanner.next::<$t>().unwrap()
    };
    ($($t:ty),+) => {
      ($(scan!($t)),+)
    };
    ($t:ty; $n:expr) => {
      scan_iter!($t; $n).collect::<Vec<_>>()
    };
    ($t_0:ty, $t_1:ty; $n:expr) => {
      scan!($t_0 = 0, $t_1 = 1; $n)
    };
    ($t_0:ty, $t_1:ty, $t_2:ty; $n:expr) => {
      scan!($t_0 = 0, $t_1 = 1, $t_2 = 2; $n)
    };
    ($($t:ty = $i:tt),+; $n:expr) => {{
      let mut vecs = ($(Vec::<$t>::with_capacity($n)),+);
      for _ in 0..$n {$(
        vecs.$i.push(scan!($t));
      )+}
      vecs
    }};
  }

  #[allow(unused_macros)]
  macro_rules! scan_iter {
    ($t:ty; $n:expr) => {
      _scanner.take::<$t>($n).map(|x| x.unwrap())
    };
  }

  #[allow(unused_macros)]
  macro_rules! print {
    ($fmt:expr) => {
      write!(_writer, $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      write!(_writer, $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! println {
    () => {
      writeln!(_writer).unwrap()
    };
    ($fmt:expr) => {
      writeln!(_writer, $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      writeln!(_writer, $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! eprint {
    ($fmt:expr) => {
      #[cfg(debug_assertions)]
      write!(::std::io::stderr(), $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      #[cfg(debug_assertions)]
      write!(::std::io::stderr(), $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! eprintln {
    () => {
      #[cfg(debug_assertions)]
      writeln!(::std::io::stderr()).unwrap()
    };
    ($fmt:expr) => {
      #[cfg(debug_assertions)]
      writeln!(::std::io::stderr(), $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      #[cfg(debug_assertions)]
      writeln!(::std::io::stderr(), $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! dump {
    ($($x:expr),+) => {
      eprint!("[{}:{}] ", file!(), line!());
      eprintln!(concat!($(stringify!($x), "={:?}; "),+), $($x),+);
    };
  }

  let table = {
    let mut vec = vec![];
    let mut sum = 0;
    for i in 1..1000 {
      sum += i;
      vec.push(sum);
    }
    vec
  };

  let (a, b) = scan!(i32, i32);
  for i in 0..table.len() - 1 {
    let j = i + 1;
    if table[j] > b && table[j] - table[i] == b - a {
      assert!(table[i] > a);
      assert_eq!(table[i] - a, table[j] - b);
      println!("{}", table[i] - a);
      return;
    }
  }
  panic!();
}

fn main() {
  let stdin = stdin();
  let stdout = stdout();
  #[cfg(debug_assertions)]
  let mut writer = stdout.lock();
  #[cfg(not(debug_assertions))]
  let mut writer = ::std::io::BufWriter::new(stdout.lock());
  solve(stdin.lock(), &mut writer);
  writer.flush().unwrap();
}

use io::Scanner;
use std::io::{stdin, stdout, BufRead, Write};

pub mod io {
  pub use self::scanner::*;

  mod scanner {
    use std::io::BufRead;
    use std::marker::PhantomData;
    use std::str::{from_utf8, FromStr};

    pub struct Scanner<R> {
      reader: R,
      buffer: Vec<u8>,
      position: usize,
    }

    impl<R: BufRead> Scanner<R> {
      pub fn new(reader: R) -> Self {
        Scanner { reader: reader, buffer: vec![], position: 0 }
      }

      pub fn next<T: Parse>(&mut self) -> Option<T> {
        Parse::parse(self.next_bytes().unwrap_or(&[]))
      }

      pub fn take<T: Parse>(&mut self, n: usize) -> Take<R, T> {
        Take { scanner: self, n: n, _marker: PhantomData }
      }

      pub fn next_bytes(&mut self) -> Option<&[u8]> {
        if self.buffer.is_empty() {
          self.read_line();
        }
        loop {
          match self.buffer.get(self.position) {
            Some(&b' ') => self.position += 1,
            Some(&b'\n') => self.read_line(),
            Some(_) => break,
            None => return None,
          }
        }
        let start = self.position;
        loop {
          match self.buffer.get(self.position) {
            Some(&b' ') | Some(&b'\n') | None => break,
            Some(_) => self.position += 1,
          }
        }
        Some(&self.buffer[start..self.position])
      }

      fn read_line(&mut self) {
        self.position = 0;
        self.buffer.clear();
        self.reader.read_until(b'\n', &mut self.buffer).unwrap();
      }
    }

    pub struct Take<'a, R: 'a, T> {
      scanner: &'a mut Scanner<R>,
      n: usize,
      _marker: PhantomData<fn() -> T>,
    }

    impl<'a, R: BufRead, T: Parse> Iterator for Take<'a, R, T> {
      type Item = Option<T>;

      fn next(&mut self) -> Option<Self::Item> {
        if self.n > 0 {
          self.n -= 1;
          Some(self.scanner.next())
        } else {
          None
        }
      }

      fn size_hint(&self) -> (usize, Option<usize>) {
        (self.n, Some(self.n))
      }
    }

    impl<'a, R: BufRead, T: Parse> ExactSizeIterator for Take<'a, R, T> {}

    pub trait Parse: Sized {
      fn parse(bytes: &[u8]) -> Option<Self>;
    }

    impl Parse for u8 {
      fn parse(bytes: &[u8]) -> Option<Self> {
        if bytes.len() == 1 {
          Some(*unsafe { bytes.get_unchecked(0) })
        } else {
          None
        }
      }
    }

    macro_rules! parse_impl {
      ($($t:ident)+) => {$(
        impl Parse for $t {
          fn parse(bytes: &[u8]) -> Option<Self> {
            from_utf8(bytes).ok().and_then(|s| $t::from_str(s).ok())
          }
        }
      )+};
    }

    parse_impl! { i32 i64 isize u32 u64 usize String }
  }
}