Problem:
Score : 300 points

Problem Statement

We will play a one-player game using a number line and N pieces.

First, we place each of these pieces at some integer coordinate.

Here, multiple pieces can be placed at the same coordinate.

Our objective is to visit all of the M coordinates X_1, X_2, ..., X_M with these pieces, by repeating the following move:

Move: Choose a piece and let x be its coordinate. Put that piece at coordinate x+1 or x-1.

Note that the coordinates where we initially place the pieces are already regarded as visited.

Find the minimum number of moves required to achieve the objective.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^5

-10^5 \leq X_i \leq 10^5

X_1, X_2, ..., X_M are all different.

Input

Input is given from Standard Input in the following format:

N M
X_1 X_2 ... X_M

Output

Find the minimum number of moves required to achieve the objective.

Sample Input 1

2 5
10 12 1 2 14

Sample Output 1

5

The objective can be achieved in five moves as follows, and this is the minimum number of moves required.

Initially, put the two pieces at coordinates 1 and 10.

Move the piece at coordinate 1 to 2.

Move the piece at coordinate 10 to 11.

Move the piece at coordinate 11 to 12.

Move the piece at coordinate 12 to 13.

Move the piece at coordinate 13 to 14.

Sample Input 2

3 7
-10 -3 0 9 -100 2 17

Sample Output 2

19

Sample Input 3

100 1
-100000

Sample Output 3

0

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

use std::io::BufRead;

pub struct StdinReader<R: BufRead> {
    pub buf_reader: R,
    pub buf: String,
}
impl<R: BufRead> StdinReader<R> {
    pub fn new(r: R) -> Self {
        StdinReader {
            buf_reader: r,
            buf: String::with_capacity(10000),
        }
    }
}

#[macro_export]
macro_rules! input {
    ($reader : expr, $t : ty) => { // one variable
        match &mut $reader {
            reader => {
                use std::io::BufRead;
                reader.buf_reader.read_line(&mut reader.buf).unwrap();
                let tmp_var: $t = reader.buf.trim().parse().unwrap(); 
                reader.buf.clear();
                tmp_var
            },
        }
    };
    ($reader : expr, $t0 : ty, $t1 : ty) => { // 2 tuple
        match &mut $reader {
            reader => {
                use std::io::BufRead;
                reader.buf_reader.read_line(&mut reader.buf).unwrap();
                let tmp_var: Vec<$t0> = reader.buf.trim().split_whitespace().map(|a| a.parse().unwrap()).collect();
                reader.buf.clear();
                (tmp_var[0], tmp_var[1])
            }
        }
    };
    ($reader : expr, $t0 : ty, $t1 : ty, $t2 : ty) => { // 3 tuple
        match &mut $reader {
            reader => {
                use std::io::BufRead;
                reader.buf_reader.read_line(&mut reader.buf).unwrap();
                let tmp_var: Vec<$t0> = reader.buf.trim().split_whitespace().map(|a| a.parse().unwrap()).collect();
                reader.buf.clear();
                (tmp_var[0], tmp_var[1], tmp_var[2])
            }
        }
    };
    ($reader : expr, $t : ty; $size : expr) => { // Vector
        match &mut $reader {
            reader => {
                use std::io::BufRead;
                reader.buf_reader.read_line(&mut reader.buf).unwrap();
                let tmp_var: $t = reader.buf.trim().split_whitespace().map(|a| a.parse().unwrap()).collect();
                reader.buf.clear();
                tmp_var
            }
        }
    };
}

use std::cmp::Ordering;
#[derive(Debug, Hash)]
struct SomeType {
    s: usize, // index of small
    l: usize, // index of large
    value: isize, // distance
}
impl Eq for SomeType {}
impl PartialEq for SomeType {
    fn eq(&self, other: &Self) -> bool {
        self.value == other.value
    }
}
impl PartialOrd for SomeType {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.value.partial_cmp(&other.value)
    }
}
impl Ord for SomeType {
    fn cmp(&self, other: &Self) -> Ordering {
        self.value.cmp(&other.value)
    }
}

fn main() {
    let handle = std::io::stdin();
    let mut r = StdinReader::new(handle.lock());

    let (n, m) = input!(r, usize, usize);
    if n >= m {
        println!("{}", 0);
        return;
    }
    let mut xs: Vec<isize> = input!(r, Vec<isize>; m);
    xs.sort();
    use std::collections::BinaryHeap;
    let mut distances: BinaryHeap<SomeType> = BinaryHeap::with_capacity(m);
    for i in 0..xs.len()-1 {
        let distance = xs[i+1] - xs[i];
        distances.push(SomeType { s: i, l: i+1, value: distance, });
    }
    use std::collections::HashSet;
    let mut hops: HashSet<(usize, usize)> = HashSet::with_capacity(n-1);
    for _ in 0..n-1 {
        let hop = distances.pop().unwrap();
        hops.insert((hop.s, hop.l));
    }
    let mut total_dis = 0;
    for i in 0..m-1 {
        let item = (i, i+1);
        if hops.contains(&item) {
            continue;
        }
        total_dis += xs[i+1]-xs[i];
    }
    println!("{}", total_dis);
}