Problem:
Score : 900 points

Problem Statement

There are N robots and M exits on a number line.
The N + M coordinates of these are all integers and all distinct.
For each i (1 \leq i \leq N), the coordinate of the i-th robot from the left is x_i.
Also, for each j (1 \leq j \leq M), the coordinate of the j-th exit from the left is y_j.

Snuke can repeatedly perform the following two kinds of operations in any order to move all the robots simultaneously:

Increment the coordinates of all the robots on the number line by 1.

Decrement the coordinates of all the robots on the number line by 1.

Each robot will disappear from the number line when its position coincides with that of an exit, going through that exit.
Snuke will continue performing operations until all the robots disappear.

When all the robots disappear, how many combinations of exits can be used by the robots?
Find the count modulo 10^9 + 7.
Here, two combinations of exits are considered different when there is a robot that used different exits in those two combinations.

Constraints

1 \leq N, M \leq 10^5

1 \leq x_1 < x_2 < ... < x_N \leq 10^9

1 \leq y_1 < y_2 < ... < y_M \leq 10^9

All given coordinates are integers.

All given coordinates are distinct.

Input

Input is given from Standard Input in the following format:

N M
x_1 x_2 ... x_N
y_1 y_2 ... y_M

Output

Print the number of the combinations of exits that can be used by the robots when all the robots disappear, modulo 10^9 + 7.

Sample Input 1

2 2
2 3
1 4

Sample Output 1

3

The i-th robot from the left will be called Robot i, and the j-th exit from the left will be called Exit j.
There are three possible combinations of exits (the exit used by Robot 1, the exit used by Robot 2) as follows:

(Exit 1, Exit 1)

(Exit 1, Exit 2)

(Exit 2, Exit 2)

Sample Input 2

3 4
2 5 10
1 3 7 13

Sample Output 2

8

The exit for each robot can be chosen independently, so there are 2^3 = 8 possible combinations of exits.

Sample Input 3

4 1
1 2 4 5
3

Sample Output 3

1

Every robot uses Exit 1.

Sample Input 4

4 5
2 5 7 11
1 3 6 9 13

Sample Output 4

6

Sample Input 5

10 10
4 13 15 18 19 20 21 22 25 27
1 5 11 12 14 16 23 26 29 30

Sample Output 5

22

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

#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::{Read, Write, BufWriter};

#[allow(dead_code)]
fn getline() -> String {
    let mut ret = String::new();
    std::io::stdin().read_line(&mut ret).ok().unwrap();
    ret
}
fn get_word() -> String {
    let mut stdin = std::io::stdin();
    let mut u8b: [u8; 1] = [0];
    loop {
        let mut buf: Vec<u8> = Vec::with_capacity(16);
        loop {
            let res = stdin.read(&mut u8b);
            if res.unwrap_or(0) == 0 || u8b[0] <= b' ' {
                break;
            } else {
                buf.push(u8b[0]);
            }
        }
        if buf.len() >= 1 {
            let ret = String::from_utf8(buf).unwrap();
            return ret;
        }
    }
}

const MOD: i64 = 1_000_000_007;
/// Refers to external ::MOD.
/// Verified by: https://beta.atcoder.jp/contests/arc099/submissions/2893648
mod mod_int {
    use ::MOD;
    use std::ops::*;
    #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
    pub struct ModInt { pub x: i64 }
    impl ModInt {
        fn check_integrity(self) {
            debug_assert!(self.x >= 0);
            debug_assert!(self.x < MOD);
        }
        // x >= 0
        pub fn new<T: Into<i64>>(x: T) -> Self { ModInt { x: x.into() % MOD } }
        #[allow(dead_code)]
        pub fn mul_fast(self, other: Self) -> Self {
            self.check_integrity();
            other.check_integrity();
            ModInt { x: self.x * other.x % MOD }
        }
        #[allow(dead_code)]
        pub fn mul_slow(self, other: Self) -> Self {
            // Naive multiplication in order to avoid overflow
            self.check_integrity();
            other.check_integrity();
            let mut sum = ModInt::new(0);
            let mut cur = self;
            let mut e = other.x;
            if self.x < other.x {
                cur = other;
                e = self.x;
            }
            while e > 0 {
                if e % 2 == 1 {
                    sum = sum + cur;
                }
                cur = cur + cur;
                e /= 2;
            }
            sum
        }
        pub fn pow(self, mut e: i64) -> Self {
            self.check_integrity();
            debug_assert!(e >= 0);
            let mut sum = ModInt::new(1);
            let mut cur = ModInt::new(self.x);
            while e > 0 {
                if e % 2 != 0 {
                    sum = sum * cur;
                }
                cur = cur * cur;
                e /= 2;
            }
            sum
        }
        #[allow(unused)]
        pub fn inv(self) -> Self { self.pow(MOD - 2) }
    }
    impl Add for ModInt {
        type Output = Self;
        fn add(self, other: Self) -> Self {
            self.check_integrity();
            other.check_integrity();
            let mut sum = self.x + other.x;
            if sum >= MOD { sum -= MOD; }
            ModInt { x: sum }
        }
    }
    impl Sub for ModInt {
        type Output = Self;
        fn sub(self, other: Self) -> Self {
            self.check_integrity();
            other.check_integrity();
            let mut sum = self.x - other.x;
            if sum < 0 { sum += MOD; }
            ModInt { x: sum }
        }
    }
    impl Mul for ModInt {
        type Output = Self;
        fn mul(self, other: Self) -> Self {
            self.mul_fast(other)
        }
    }
    impl AddAssign for ModInt {
        fn add_assign(&mut self, rhs: ModInt) { *self = *self + rhs; }
    }
    impl SubAssign for ModInt {
        fn sub_assign(&mut self, rhs: ModInt) { *self = *self - rhs; }
    }
    impl MulAssign for ModInt {
        fn mul_assign(&mut self, rhs: ModInt) { *self = *self * rhs; }
    }
    impl ::std::fmt::Display for ModInt {
        fn fmt(&self, f: &mut::std::fmt::Formatter) -> ::std::fmt::Result {
            self.x.fmt(f)
        }
    }
} // mod mod_int
use mod_int::*;

const INF: i64 = 1 << 40;

#[allow(dead_code)]
fn get<T: std::str::FromStr>() -> T { get_word().parse().ok().unwrap() }

/**
 * Segment Tree. This data structure is useful for fast folding on intervals of an array
 * whose elements are elements of monoid I. Note that constructing this tree requires the identity
 * element of I and the operation of I.
 * Verified by: yukicoder No. 259 (http://yukicoder.me/submissions/100581)
 *              AGC015-E (http://agc015.contest.atcoder.jp/submissions/1461001)
 */
struct SegTree<I, BiOp> {
    n: usize,
    dat: Vec<I>,
    op: BiOp,
    e: I,
}

impl<I, BiOp> SegTree<I, BiOp>
    where BiOp: Fn(I, I) -> I,
          I: Copy {
    pub fn new(n_: usize, op: BiOp, e: I) -> Self {
        let mut n = 1;
        while n < n_ { n *= 2; } // n is a power of 2
        SegTree {n: n, dat: vec![e; 2 * n - 1], op: op, e: e}
    }
    /* ary[k] <- v */
    pub fn update(&mut self, idx: usize, v: I) {
        let mut k = idx + self.n - 1;
        self.dat[k] = v;
        while k > 0 {
            k = (k - 1) / 2;
            self.dat[k] = (self.op)(self.dat[2 * k + 1], self.dat[2 * k + 2]);
        }
    }
    /* [a, b) (note: half-inclusive)
     * http://proc-cpuinfo.fixstars.com/2017/07/optimize-segment-tree/ */
    pub fn query(&self, mut a: usize, mut b: usize) -> I {
        let mut left = self.e;
        let mut right = self.e;
        a += self.n - 1;
        b += self.n - 1;
        while a < b {
            if (a & 1) == 0 {
                left = (self.op)(left, self.dat[a]);
            }
            if (b & 1) == 0 {
                right = (self.op)(self.dat[b - 1], right);
            }
            a = a / 2;
            b = (b - 1) / 2;
        }
        (self.op)(left, right)
    }
}


fn solve() {
    let out = std::io::stdout();
    let mut out = BufWriter::new(out.lock());
    macro_rules! puts {
        ($format:expr) => (write!(out,$format).unwrap());
        ($format:expr, $($args:expr),+) => (write!(out,$format,$($args),*).unwrap())
    }
    let n: usize = get();
    let m: usize = get();
    let x: Vec<i64> = (0 .. n).map(|_| get()).collect();
    let y: Vec<i64> = (0 .. m).map(|_| get()).collect();
    let mut pq = Vec::new();
    let mut ps = Vec::new();
    let mut qs_p = Vec::new();
    for i in 0 .. n {
        let idx = y.binary_search(&x[i]).err().unwrap();
        let p = if idx >= 1 { x[i] - y[idx - 1] } else { INF };
        let q = if idx <= m - 1 { y[idx] - x[i] } else { INF };
        if p != INF && q != INF {
            pq.push((p, q));
            ps.push(p);
            qs_p.push(q);
        }
    }
    ps.sort();
    ps.dedup();
    qs_p.push(-INF);
    qs_p.sort();
    qs_p.dedup();
    pq.sort();
    let mut qs = vec![Vec::new(); ps.len()];
    for &(p, q) in pq.iter() {
        let idx = ps.binary_search(&p).ok().unwrap();
        qs[idx].push(q);
    }
    let nn = qs_p.len();
    let mut st = SegTree::new(nn, |x, y| x + y, ModInt::new(0));
    st.update(0, ModInt::new(1));
    for qs in qs.iter() {
        let mut delay = Vec::new();
        for &q in qs.iter() {
            let idx = qs_p.binary_search(&q).ok().unwrap();
            delay.push((idx, st.query(0, idx + 1)));
        }
        for (idx, val) in delay {
            st.update(idx, val);
        }
    }
    puts!("{}\n", st.query(0, nn));
}

fn main() {
    // In order to avoid potential stack overflow, spawn a new thread.
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    thd.spawn(|| solve()).unwrap().join().unwrap();
}