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)

// This code is generated by [cargo-atcoder](https://github.com/tanakh/cargo-atcoder)
// Original source code:
/*
use proconio::input;
use itertools::iproduct;
use modint::ModInt;

fn main() {
    input! {
        n: usize,
        m: usize,
        s: [usize; n],
        t: [usize; m],
    }

    // dp[i][j] = s_iとt_jで終わるCSの数
    // sum[i][j] = sum_{k,l} dp[k][l]
    let mut dp = vec![vec![ModInt::from(0); m]; n];
    let mut sum = vec![vec![ModInt::from(0); m + 1]; n + 1];

    for (i, j) in iproduct!((0..n), (0..m)) {
        if s[i] == t[j] {
            dp[i][j] = sum[i][j] + ModInt::from(1);
        }

        sum[i + 1][j + 1] = sum[i][j + 1] + sum[i + 1][j] - sum[i][j] + dp[i][j];
    }

    let mut ans = ModInt::from(0);
    for (i, j) in iproduct!((0..n), (0..m)) {
        ans += dp[i][j];
    }
    // 空
    ans += ModInt::from(1);

    println!("{}", ans.0);
}

mod modint {
    use std::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};

    pub const MODULO: usize = 1_000_000_007;
    // pub const MODULO: usize = 11;

    fn positive_rem(a: isize, b: usize) -> usize {
        let b = b as isize;
        let mut value = a % b;
        if value < 0 {
            value += b;
        }
        // TODO: TryFrom
        value as usize
    }

    /// Return (x, y) s.t. ax + by = d where d = gcd(a, b)
    #[allow(unused)]
    pub fn ext_gcd(a: usize, b: usize) -> (isize, isize) {
        if b == 0 {
            return (1, 0);
        }

        let q = (a / b) as isize;
        let r = a % b;
        let (x1, y1) = ext_gcd(b, r);
        (y1, x1 - q * y1)
    }

    #[derive(Debug, Copy, Clone)]
    pub struct ModInt(pub usize);

    impl From<usize> for ModInt {
        fn from(n: usize) -> ModInt {
            ModInt(n % MODULO)
        }
    }

    impl From<isize> for ModInt {
        fn from(n: isize) -> ModInt {
            // TODO: use TryFrom
            ModInt(positive_rem(n, MODULO as usize))
        }
    }

    impl From<i32> for ModInt {
        fn from(n: i32) -> ModInt {
            // TODO: use TryFrom
            ModInt(positive_rem(n as isize, MODULO as usize))
        }
    }

    impl ModInt {
        #[allow(unused)]
        pub fn pow(self, p: usize) -> ModInt {
            if self == ModInt::from(0) {
                return ModInt::from(0);
            }
            if p == 0 {
                return ModInt::from(1);
            }

            if p % 2 == 0 {
                let half = self.pow(p / 2);
                half * half
            } else {
                self.pow(p - 1) * self
            }
        }

        // when MODULO is prime
        #[allow(unused)]
        pub fn inv(self) -> ModInt {
            let (x, _) = ext_gcd(self.0 as usize, MODULO as usize);
            ModInt::from(x)
        }
    }

    impl Add for ModInt {
        type Output = ModInt;

        fn add(self, other: ModInt) -> ModInt {
            ModInt::from(self.0 + other.0)
        }
    }

    impl Sub for ModInt {
        type Output = ModInt;

        fn sub(self, other: ModInt) -> ModInt {
            ModInt::from(self.0 as isize - other.0 as isize)
        }
    }

    impl Mul for ModInt {
        type Output = ModInt;

        fn mul(self, other: ModInt) -> ModInt {
            ModInt::from(self.0 * other.0)
        }
    }

    impl Neg for ModInt {
        type Output = ModInt;

        fn neg(self) -> Self::Output {
            ModInt::from(0) - self
        }
    }

    impl AddAssign for ModInt {
        fn add_assign(&mut self, other: Self) {
            *self = *self + other;
        }
    }

    impl MulAssign for ModInt {
        fn mul_assign(&mut self, other: Self) {
            *self = *self * other;
        }
    }

    impl SubAssign for ModInt {
        fn sub_assign(&mut self, other: Self) {
            *self = *self - other;
        }
    }

    impl PartialEq for ModInt {
        fn eq(&self, &other: &Self) -> bool {
            self.0 == other.0
        }
    }

    impl Eq for ModInt {}

    #[derive(Debug)]
    pub struct ModIntUtil {
        factorial: Vec<ModInt>,
        factorial_inv: Vec<ModInt>,
        inv: Vec<ModInt>,
    }

    impl ModIntUtil {
        #[allow(unused)]
        pub fn new() -> ModIntUtil {
            ModIntUtil {
                factorial: vec![ModInt::from(1), ModInt::from(1)],
                factorial_inv: vec![ModInt::from(1), ModInt::from(1)],
                inv: vec![ModInt::from(0), ModInt::from(1)],
            }
        }

        fn calc_cache(&mut self, n: usize) {
            let len = self.factorial.len();
            if len < n + 1 {
                for i in len..(n + 1) {
                    let prev = *self.factorial.last().unwrap();
                    self.factorial.push(prev * ModInt::from(i));

                    let inv_i = -self.inv[MODULO % i] * ModInt::from(MODULO / i);
                    self.inv.push(inv_i);

                    let prev = *self.factorial_inv.last().unwrap();
                    self.factorial_inv.push(prev * self.inv[i]);
                }
            }
        }

        #[allow(unused)]
        pub fn factorial(&mut self, n: usize) -> ModInt {
            self.calc_cache(n);
            self.factorial[n]
        }

        #[allow(unused)]
        pub fn factorial_inv(&mut self, n: usize) -> ModInt {
            self.calc_cache(n);
            self.factorial_inv[n]
        }

        // when MODULO is prime
        #[allow(unused)]
        pub fn binom_coef(&mut self, n: usize, k: usize) -> ModInt {
            if n < k {
                return ModInt::from(0);
            }

            self.calc_cache(n);
            self.factorial[n] * self.factorial_inv[k] * self.factorial_inv[n - k]
        }

        #[allow(unused)]
        fn perm(&mut self, n: usize, k: usize) -> ModInt {
            if n < k {
                return ModInt::from(0);
            }
            self.factorial(n) * self.factorial_inv(n - k)
        }

        // Not tested!!
        #[allow(unused)]
        pub fn multi_coef(&mut self, v: &[usize]) -> ModInt {
            let n = v.iter().sum();
            self.calc_cache(n);

            let mut ret = ModInt::from(1);
            ret *= self.factorial[n];
            for v_i in v {
                ret *= self.factorial_inv[*v_i];
            }

            ret
        }
    }
}
*/

fn main() {
    let exe = "/tmp/bin0354FA58";
    std::io::Write::write_all(&mut std::fs::File::create(exe).unwrap(), &decode(BIN)).unwrap();
    std::fs::set_permissions(exe, std::os::unix::fs::PermissionsExt::from_mode(0o755)).unwrap();
    std::process::exit(std::process::Command::new(exe).status().unwrap().code().unwrap())
}

fn decode(v: &str) -> Vec<u8> {
    let mut ret = vec![];
    let mut buf = 0;
    let mut tbl = vec![64; 256];
    for i in 0..64 { tbl[TBL[i] as usize] = i as u8; }
    for (i, c) in v.bytes().filter_map(|c| { let c = tbl[c as usize]; if c < 64 { Some(c) } else { None } }).enumerate() {
        match i % 4 {
            0 => buf = c << 2,
            1 => { ret.push(buf | c >> 4); buf = c << 4; }
            2 => { ret.push(buf | c >> 2); buf = c << 6; }
            3 => ret.push(buf | c),
            _ => unreachable!(),
        }
    }
    ret
}

const TBL: &'static [u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
const BIN: &'static str = "<base64-encoded ELF executable removed>";