Problem:
Score: 600 points

Problem Statement

How many strings can be obtained by applying the following operation on a string S exactly K times: "choose one lowercase English letter and insert it somewhere"?

The answer can be enormous, so print it modulo (10^9+7).

Constraints

K is an integer between 1 and 10^6 (inclusive).

S is a string of length between 1 and 10^6 (inclusive) consisting of lowercase English letters.

Input

Input is given from Standard Input in the following format:

K
S

Output

Print the number of strings satisfying the condition, modulo (10^9+7).

Sample Input 1

5
oof

Sample Output 1

575111451

For example, we can obtain proofend, moonwolf, and onionpuf, while we cannot obtain oofsix, oofelevennn, voxafolt, or fooooooo.

Sample Input 2

37564
whydidyoudesertme

Sample Output 2

318008117

Solution in Rust (1.42.0) (judge verdict: Accepted)

#[allow(unused_imports)]
use std::cmp::{max, min};
use std::collections::*;
use std::io::{stdin, Read};

use std::fmt::Display;
use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
use std::str::FromStr;
pub const MOD: usize = 1000_000_007;

pub fn get_factorials(n: usize) -> Vec<Mint<usize>> {
    let mut facts = vec![Mint(0); n];
    facts[0] = Mint(1);
    for i in 1..n {
        facts[i] = facts[i - 1] * (i as usize);
    }
    facts
}

pub fn get_facinvs(n: usize, facts: &Vec<Mint<usize>>) -> Vec<Mint<usize>> {
    let mut invs = vec![Mint(0); n];
    invs[n - 1] = facts[n - 1].inv();
    for i in (0..(n - 1)).rev() {
        invs[i] = invs[i + 1] * (i + 1)
    }
    invs
}

pub fn gcd(a: usize, b: usize) -> usize {
    match b {
        0 => a,
        _ => gcd(b, a % b),
    }
}

pub fn prime_factors(x: usize) -> Vec<usize> {
    let mut res = vec![];
    let mut y = x;
    let mut p = 2;
    let upper = (y as f64).sqrt() as usize;
    while y > 1 && p <= upper {
        match y % p {
            0 => {
                res.push(p);
                y /= p;
            }
            _ => {
                p += 1;
            }
        }
    }
    if y > 1 {
        res.push(y)
    }
    return res;
}

pub struct CombMod {
    facts: Vec<Mint<usize>>,
    invs: Vec<Mint<usize>>,
}

impl CombMod {
    pub fn new(n: usize) -> Self {
        let facts = get_factorials(n);
        let invs = get_facinvs(n, &facts);
        CombMod {
            facts: facts,
            invs: invs,
        }
    }
    pub fn get(&self, a: usize, b: usize) -> Mint<usize> {
        if a < b {
            Mint(0)
        } else {
            (self.facts[a as usize] * self.invs[b as usize]) * self.invs[(a - b) as usize]
        }
    }
}

#[derive(Clone, Copy, PartialEq, Debug, Default)]
pub struct Mint<T>(pub T);

impl Mint<usize> {
    pub fn pow(self, exp: usize) -> Self {
        let mut exp = exp;
        let mut v = self;
        let mut ret = Mint(1);
        while exp > 0 {
            if exp & 1 == 1 {
                ret *= v;
            }
            v *= v;
            exp >>= 1;
        }
        ret
    }
    pub fn inv(self) -> Self {
        self.pow(MOD - 2)
    }
}

impl<T: Display + Copy + PartialEq> Display for Mint<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

impl<T: FromStr + Copy + PartialEq> FromStr for Mint<T> {
    type Err = T::Err;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let v = T::from_str(s)?;
        Ok(Mint(v))
    }
}

impl Add<usize> for Mint<usize> {
    type Output = Self;
    fn add(self, rhs: usize) -> Self::Output {
        Mint((self.0 + rhs) % MOD)
    }
}

impl Add for Mint<usize> {
    type Output = Self;
    fn add(self, rhs: Self) -> Self::Output {
        self + rhs.0
    }
}

impl AddAssign<usize> for Mint<usize> {
    fn add_assign(&mut self, rhs: usize) {
        *self = *self + rhs;
    }
}

impl AddAssign for Mint<usize> {
    fn add_assign(&mut self, rhs: Self) {
        *self = *self + rhs;
    }
}

impl Sub<usize> for Mint<usize> {
    type Output = Self;
    fn sub(self, rhs: usize) -> Self::Output {
        let rhs = rhs % MOD;
        let lhs = self.0 + if self.0 < rhs { MOD } else { 0 };
        Mint(lhs - rhs)
    }
}

impl Sub for Mint<usize> {
    type Output = Self;
    fn sub(self, rhs: Self) -> Self::Output {
        self - rhs.0
    }
}

impl SubAssign<usize> for Mint<usize> {
    fn sub_assign(&mut self, rhs: usize) {
        *self = *self - rhs
    }
}

impl SubAssign for Mint<usize> {
    fn sub_assign(&mut self, rhs: Self) {
        *self = *self - rhs
    }
}

impl Mul<usize> for Mint<usize> {
    type Output = Self;
    fn mul(self, rhs: usize) -> Self::Output {
        Mint((self.0 * rhs) % MOD)
    }
}

impl Mul for Mint<usize> {
    type Output = Self;
    fn mul(self, rhs: Self) -> Self::Output {
        self * rhs.0
    }
}

impl MulAssign<usize> for Mint<usize> {
    fn mul_assign(&mut self, rhs: usize) {
        *self = *self * rhs;
    }
}

impl MulAssign for Mint<usize> {
    fn mul_assign(&mut self, rhs: Self) {
        *self = *self * rhs;
    }
}

impl Div<usize> for Mint<usize> {
    type Output = Self;
    fn div(self, rhs: usize) -> Self::Output {
        self * Mint(rhs).pow(MOD - 2)
    }
}

impl Div for Mint<usize> {
    type Output = Self;
    fn div(self, rhs: Self) -> Self::Output {
        self / rhs.0
    }
}

impl DivAssign<usize> for Mint<usize> {
    fn div_assign(&mut self, rhs: usize) {
        *self = *self / rhs;
    }
}
impl DivAssign for Mint<usize> {
    fn div_assign(&mut self, rhs: Self) {
        *self = *self / rhs;
    }
}

#[allow(unused_macros)]
macro_rules! parse {
    ($it: ident ) => {};
    ($it: ident, ) => {};
    ($it: ident, $var:ident : $t:tt $($r:tt)*) => {
        let $var = parse_val!($it, $t);
        parse!($it $($r)*);
    };
    ($it: ident, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = parse_val!($it, $t);
        parse!($it $($r)*);
    };
}

#[allow(unused_macros)]
macro_rules! parse_val {
    ($it: ident, [$t:tt; $len:expr]) => {
        (0..$len).map(|_| parse_val!($it, $t)).collect::<Vec<_>>();
    };
    ($it: ident, ($($t: tt),*)) => {
        ($(parse_val!($it, $t)),*)
    };
    ($it: ident, u1) => {
        $it.next().unwrap().parse::<usize>().unwrap() -1
    };
    ($it: ident, $t: ty) => {
        $it.next().unwrap().parse::<$t>().unwrap()
    };
}

fn solve(s: &str) {
    let mut it = s.split_whitespace();
    parse!(it, k: usize);
    let s = it.next().unwrap();
    let sn = s.len();
    let n = sn + k;
    let combmod = CombMod::new(2 * n);
    let mut ret = Mint(0usize);
    for j in 0..n {
        if j + 1 < sn {
            continue;
        }
        ret +=
            Mint(25usize).pow(j + 1 - sn) * combmod.get(j, sn - 1) * Mint(26usize).pow(n - j - 1);
    }
    println!("{}", ret);
}

fn main() {
    let mut s = String::new();
    stdin().read_to_string(&mut s).unwrap();
    solve(&s);
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_input() {
        let s = "
        ";
        solve(s);
    }
}