Problem:
Score : 500 points

Problem Statement

You are given two integer sequences S and T of length N and M, respectively, both consisting of integers between 1 and 10^5 (inclusive).

In how many pairs of a subsequence of S and a subsequence of T do the two subsequences are the same in content?

Here the subsequence of A is a sequence obtained by removing zero or more elements from A and concatenating the remaining elements without changing the order.

For both S and T, we distinguish two subsequences if the sets of the indices of the removed elements are different, even if the subsequences are the same in content.

Since the answer can be tremendous, print the number modulo 10^9+7.

Constraints

1 \leq N, M \leq 2 \times 10^3

The length of S is N.

The length of T is M.

1 \leq S_i, T_i \leq 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
S_1 S_2 ... S_{N-1} S_{N}
T_1 T_2 ... T_{M-1} T_{M}

Output

Print the number of pairs of a subsequence of S and a subsequence of T such that the subsequences are the same in content, modulo 10^9+7.

Sample Input 1

2 2
1 3
3 1

Sample Output 1

3

S has four subsequences: (), (1), (3), (1, 3).

T has four subsequences: (), (3), (1), (3, 1).

There are 1 \times 1 pair of subsequences in which the subsequences are both (), 1 \times 1 pair of subsequences in which the subsequences are both (1), and 1 \times 1 pair of subsequences in which the subsequences are both (3), for a total of three pairs.

Sample Input 2

2 2
1 1
1 1

Sample Output 2

6

S has four subsequences: (), (1), (1), (1, 1).

T has four subsequences: (), (1), (1), (1, 1).

There are 1 \times 1 pair of subsequences in which the subsequences are both (), 2 \times 2 pairs of subsequences in which the subsequences are both (1), and 1 \times 1 pair of subsequences in which the subsequences are both (1,1), for a total of six pairs.
Note again that we distinguish two subsequences if the sets of the indices of the removed elements are different, even if the subsequences are the same in content.

Sample Input 3

4 4
3 4 5 6
3 4 5 6

Sample Output 3

16

Sample Input 4

10 9
9 6 5 7 5 9 8 5 6 7
8 6 8 5 5 7 9 9 7

Sample Output 4

191

Sample Input 5

20 20
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Sample Output 5

846527861

Be sure to print the number modulo 10^9+7.

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

macro_rules ! input { ( source = $ s : expr , $ ( $ r : tt ) * ) => { let mut iter = $ s . split_whitespace ( ) ; input_inner ! { iter , $ ( $ r ) * } } ; ( iter = $ iter : ident , $ ( $ r : tt ) * ) => { let s = { use std :: io :: Read ; let mut s = String :: new ( ) ; std :: io :: stdin ( ) . read_to_string ( & mut s ) . unwrap ( ) ; s } ; let mut $ iter = s . split_whitespace ( ) ; input_inner ! { $ iter , $ ( $ r ) * } } ; ( $ ( $ r : tt ) * ) => { let s = { use std :: io :: Read ; let mut s = String :: new ( ) ; std :: io :: stdin ( ) . read_to_string ( & mut s ) . unwrap ( ) ; s } ; let mut iter = s . split_whitespace ( ) ; input_inner ! { iter , $ ( $ r ) * } } ; }
macro_rules ! input_inner { ( $ iter : expr ) => { } ; ( $ iter : expr , ) => { } ; ( $ iter : expr , mut $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let mut $ var = read_value ! ( $ iter , $ t ) ; input_inner ! { $ iter $ ( $ r ) * } } ; ( $ iter : expr , mut $ var : ident $ ( $ r : tt ) * ) => { input_inner ! { $ iter , mut $ var : usize $ ( $ r ) * } } ; ( $ iter : expr , $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let $ var = read_value ! ( $ iter , $ t ) ; input_inner ! { $ iter $ ( $ r ) * } } ; ( $ iter : expr , $ var : ident $ ( $ r : tt ) * ) => { input_inner ! { $ iter , $ var : usize $ ( $ r ) * } } ; }
macro_rules ! read_value { ( $ iter : expr , ( $ ( $ t : tt ) ,* ) ) => { ( $ ( read_value ! ( $ iter , $ t ) ) ,* ) } ; ( $ iter : expr , [ $ t : tt ; $ len : expr ] ) => { ( 0 ..$ len ) . map ( | _ | read_value ! ( $ iter , $ t ) ) . collect ::< Vec < _ >> ( ) } ; ( $ iter : expr , { chars : $ base : expr } ) => { read_value ! ( $ iter , String ) . chars ( ) . map ( | c | ( c as u8 - $ base as u8 ) as usize ) . collect ::< Vec < usize >> ( ) } ; ( $ iter : expr , { char : $ base : expr } ) => { read_value ! ( $ iter , { chars : $ base } ) [ 0 ] } ; ( $ iter : expr , chars ) => { read_value ! ( $ iter , String ) . chars ( ) . collect ::< Vec < char >> ( ) } ; ( $ iter : expr , char ) => { read_value ! ( $ iter , chars ) [ 0 ] } ; ( $ iter : expr , usize1 ) => { read_value ! ( $ iter , usize ) - 1 } ; ( $ iter : expr , $ t : ty ) => { $ iter . next ( ) . unwrap ( ) . parse ::<$ t > ( ) . unwrap ( ) } ; }
const MOD: i64 = 1_000_000_007;
#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct Modi64(pub i64);
impl Modi64 {
    #[inline]
    pub fn new(i: i64) -> Modi64 {
        Modi64(Modi64::modulo(i, MOD))
    }
    #[inline]
    pub fn modulo(i: i64, m: i64) -> i64 {
        if i < 0 {
            i % m + m
        } else {
            i % m
        }
    }
    pub fn pow(self, y: i64) -> Modi64 {
        let mut y = Modi64(Modi64::modulo(y, MOD - 1));
        let mut x = 1.into();
        let mut base = self;
        while y.0 > 0 {
            if y.0 & 1 == 1 {
                x *= base;
            }
            base *= base;
            y = Modi64(y.0 >> 1);
        }
        x
    }
    #[inline]
    pub fn inv(self) -> Modi64 {
        self.pow(MOD - 2)
    }
}
impl From<i64> for Modi64 {
    fn from(i: i64) -> Modi64 {
        Modi64::new(i)
    }
}
impl std::ops::Add for Modi64 {
    type Output = Modi64;
    fn add(self, rhs: Modi64) -> Modi64 {
        (self.0 + rhs.0).into()
    }
}
impl std::ops::AddAssign for Modi64 {
    fn add_assign(&mut self, rhs: Modi64) {
        *self = *self + rhs;
    }
}
impl std::ops::Sub for Modi64 {
    type Output = Modi64;
    fn sub(self, rhs: Modi64) -> Modi64 {
        (self.0 - rhs.0).into()
    }
}
impl std::ops::SubAssign for Modi64 {
    fn sub_assign(&mut self, rhs: Modi64) {
        *self = *self - rhs;
    }
}
impl std::ops::Mul for Modi64 {
    type Output = Modi64;
    fn mul(self, rhs: Modi64) -> Modi64 {
        (self.0 * rhs.0).into()
    }
}
impl std::ops::MulAssign for Modi64 {
    fn mul_assign(&mut self, rhs: Modi64) {
        *self = *self * rhs;
    }
}
impl std::ops::Div for Modi64 {
    type Output = Modi64;
    fn div(self, rhs: Modi64) -> Modi64 {
        self * rhs.inv()
    }
}
impl std::ops::DivAssign for Modi64 {
    fn div_assign(&mut self, rhs: Modi64) {
        *self = *self / rhs;
    }
}
impl std::ops::Neg for Modi64 {
    type Output = Modi64;
    fn neg(self) -> Modi64 {
        (-self.0).into()
    }
}
impl std::iter::Sum for Modi64 {
    fn sum<I: Iterator<Item = Modi64>>(iter: I) -> Modi64 {
        iter.fold(0.into(), std::ops::Add::add)
    }
}
impl<'a> std::iter::Sum<&'a Modi64> for Modi64 {
    fn sum<I: Iterator<Item = &'a Modi64>>(iter: I) -> Modi64 {
        iter.fold(0.into(), |x, &y| x + y)
    }
}
#[inline]
fn rmb(k: usize) -> usize {
    (k as isize & -(k as isize)) as usize
}
pub trait Monoid {
    type T: Clone + PartialEq;
    fn mempty(&self) -> Self::T;
    fn mappend(&self, x: &Self::T, y: &Self::T) -> Self::T;
}
pub struct BinaryIndexedTree2D<M: Monoid> {
    h: usize,
    w: usize,
    bit: Vec<Vec<M::T>>,
    m: M,
}
impl<M: Monoid> BinaryIndexedTree2D<M> {
    pub fn new(h: usize, w: usize, m: M) -> BinaryIndexedTree2D<M> {
        let bit = vec![vec![m.mempty(); w + 1]; h + 1];
        BinaryIndexedTree2D {
            h: h,
            w: w,
            bit: bit,
            m: m,
        }
    }
    #[doc = " 1-indexed"]
    pub fn query(&self, i: usize, j: usize) -> M::T {
        let mut res = self.m.mempty();
        let mut a = i;
        while a > 0 {
            let mut b = j;
            while b > 0 {
                res = self.m.mappend(&res, &self.bit[a][b]);
                b -= rmb(b);
            }
            a -= rmb(a);
        }
        res
    }
    #[doc = " 1-indexed"]
    pub fn update(&mut self, i: usize, j: usize, x: M::T) {
        let mut a = i;
        while a <= self.h {
            let mut b = j;
            while b <= self.w {
                self.bit[a][b] = self.m.mappend(&self.bit[a][b], &x);
                b += rmb(b);
            }
            a += rmb(a);
        }
    }
}
pub struct AnyMonoid<T: Clone, F: Fn(&T, &T) -> T> {
    id: T,
    op: F,
}
impl<T: Clone, F: Fn(&T, &T) -> T> AnyMonoid<T, F> {
    pub fn new(id: T, op: F) -> AnyMonoid<T, F> {
        AnyMonoid { id: id, op: op }
    }
}
impl<T: Clone + PartialEq, F: Fn(&T, &T) -> T> Monoid for AnyMonoid<T, F> {
    type T = T;
    fn mempty(&self) -> T {
        self.id.clone()
    }
    fn mappend(&self, x: &T, y: &T) -> T {
        (self.op)(x, y)
    }
}
fn main() {
    input! { n, m, s: [usize; n], t: [usize; m] };
    let mut bit = BinaryIndexedTree2D::new(n, m, AnyMonoid::new(Modi64(0), |&x, &y| x + y));
    for i in 0..n {
        for j in 0..m {
            if s[i] == t[j] {
                let x = bit.query(i, j);
                bit.update(i + 1, j + 1, x + 1.into());
            }
        }
    }
    println!("{}", (bit.query(n, m) + 1.into()).0);
}