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 PyPy3 (7.3.0) (judge verdict: Accepted)

k = int(input())
s = input()
n = len(s)
mod = 10 ** 9 + 7

f = [1, 1]
f_inv = [1, 1]
inv = [0, 1]
 
for i in range(2, n+k+1):
    f.append((f[-1] * i ) % mod)
    inv.append((-inv[mod % i] * (mod // i) ) % mod)
    f_inv.append((f_inv[-1] * inv[-1]) % mod)

def comb(n, k):
    return f[n] * f_inv[k] * f_inv[n-k] % mod

ans = 0
for i in range(k+1):
    x = pow(25, i, mod) * pow(26, k-i, mod) * comb(n-1+i, n-1)
    ans += x
    ans %= mod

print(ans)