Buggy python submission (verdict: Runtime Error):
import sys
import numpy as np
readline = sys.stdin.buffer.readline

MOD = 10 ** 9 + 7


def main(K, N, MOD):
    def cumprod(A, mod=MOD):
        L = len(A)
        Lsq = int(L**.5 + 1)
        A = np.resize(A, Lsq**2).reshape(Lsq, Lsq)
        for n in range(1, Lsq):
            A[:, n] *= A[:, n - 1]
            A[:, n] %= mod
        for n in range(1, Lsq):
            A[n] *= A[n - 1, -1]
            A[n] %= mod
        return A.ravel()[:L]

    def make_fact(U, mod=MOD):
        x = np.arange(U, dtype=np.int64)
        x[0] = 1
        fact = cumprod(x, mod)
        x = np.arange(U, 0, -1, dtype=np.int64)
        x[0] = pow(int(fact[-1]), mod - 2, mod)
        fact_inv = cumprod(x, mod)[::-1]
        fact.flags.writeable = False
        fact_inv.flags.writeable = False
        return fact, fact_inv

    fac, finv = make_fact(2_000_010, MOD)
    f = np.zeros(K + 1, np.int64)
    f = fac[N - 1:K + N] * finv[N - 1] % MOD * finv[:K + 1] % MOD

    x = 1
    for i in range(1, K + 1):
        x = (x * 25) % MOD
        f[i] = f[i] * x % MOD
    for i in range(1, K + 1):
        f[i] += f[i - 1] * 26
        f[i] %= MOD
    return f[-1]


if sys.argv[-1] == 'ONLINE_JUDGE':
    import numba
    from numba.pycc import CC
    i8 = numba.from_dtype(np.int64)
    signature = (i8, i8, i8)
    cc = CC('my_module')
    cc.export('main', signature)(main)
    cc.compile()

from my_module import main
K = int(readline())
N = len(readline())
print(main(K, N, MOD))


Fixed submission:
import sys
import numpy as np

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

MOD = 10**9 + 7

def main(K, N, MOD):
    def fact_table(N, MOD):
        inv = np.empty(N, np.int64)
        inv[0] = 0
        inv[1] = 1
        for n in range(2, N):
            q, r = divmod(MOD, n)
            inv[n] = inv[r] * (-q) % MOD
        fact = np.empty(N, np.int64)
        fact[0] = 1
        for n in range(1, N):
            fact[n] = n * fact[n - 1] % MOD
        fact_inv = np.empty(N, np.int64)
        fact_inv[0] = 1
        for n in range(1, N):
            fact_inv[n] = fact_inv[n - 1] * inv[n] % MOD
        return fact, fact_inv, inv

    fact, fact_inv, inv = fact_table(2_000_010, MOD)
    f = np.zeros(K + 1, np.int64)
    f = fact[N - 1:K + N] * fact_inv[N - 1] % MOD * fact_inv[0:K + 1] % MOD

    x = 1
    for i in range(1, K + 1):
        x = (x * 25) % MOD
        f[i] = f[i] * x % MOD
    for i in range(1, K + 1):
        f[i] += f[i - 1] * 26
        f[i] %= MOD
    return f[-1]

if sys.argv[-1] == 'ONLINE_JUDGE':  #　なにそれ
    import numba
    from numba.pycc import CC
    i8 = numba.from_dtype(np.int64)
    signature = (i8, i8, i8)

    cc = CC('my_module')
    cc.export('main', signature)(main)
    cc.compile()


from my_module import main
K = int(readline())
N = len(read().rstrip())

print(main(K, N, MOD))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
import numpy as np
readline = sys.stdin.readline

MOD = 10 ** 9 + 7


def main():
    def fact_table(N, MOD):
        inv = np.empty(N, np.int64)
        inv[0] = 0
        inv[1] = 1
        for n in range(2, N):
            q, r = divmod(MOD, n)
            inv[n] = inv[r] * (-q) % MOD
        fact = np.empty(N, np.int64)
        fact[0] = 1
        for n in range(1, N):
            fact[n] = n * fact[n - 1] % MOD
        fact_inv = np.empty(N, np.int64)
        fact_inv[0] = 1
        for n in range(1, N):
            fact_inv[n] = fact_inv[n - 1] * inv[n] % MOD
        return fact, fact_inv, inv

    fac, finv, inv = fact_table(2_000_010, MOD)
    K = int(readline())
    N = len(readline())
    f = np.zeros(K + 1, np.int64)
    f = fac[N - 1:K + N] * finv[N - 1] % MOD * finv[:K + 1] % MOD

    x = 1
    for i in range(1, K + 1):
        x = (x * 25) % MOD
        f[i] = f[i] * x % MOD
    for i in range(1, K + 1):
        f[i] += f[i - 1] * 26
        f[i] %= MOD

    print(f[-1])


import numba
from numba.pycc import CC
i8 = numba.from_dtype(np.int64)
signature = (i8, i8, i8)

main()


Fixed submission:
import sys
import numpy as np

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

MOD = 10**9 + 7

def main(K, N, MOD):
    def fact_table(N, MOD):
        inv = np.empty(N, np.int64)
        inv[0] = 0
        inv[1] = 1
        for n in range(2, N):
            q, r = divmod(MOD, n)
            inv[n] = inv[r] * (-q) % MOD
        fact = np.empty(N, np.int64)
        fact[0] = 1
        for n in range(1, N):
            fact[n] = n * fact[n - 1] % MOD
        fact_inv = np.empty(N, np.int64)
        fact_inv[0] = 1
        for n in range(1, N):
            fact_inv[n] = fact_inv[n - 1] * inv[n] % MOD
        return fact, fact_inv, inv

    fact, fact_inv, inv = fact_table(2_000_010, MOD)
    f = np.zeros(K + 1, np.int64)
    f = fact[N - 1:K + N] * fact_inv[N - 1] % MOD * fact_inv[0:K + 1] % MOD

    x = 1
    for i in range(1, K + 1):
        x = (x * 25) % MOD
        f[i] = f[i] * x % MOD
    for i in range(1, K + 1):
        f[i] += f[i - 1] * 26
        f[i] %= MOD
    return f[-1]

if sys.argv[-1] == 'ONLINE_JUDGE':  #　なにそれ
    import numba
    from numba.pycc import CC
    i8 = numba.from_dtype(np.int64)
    signature = (i8, i8, i8)

    cc = CC('my_module')
    cc.export('main', signature)(main)
    cc.compile()


from my_module import main
K = int(readline())
N = len(read().rstrip())

print(main(K, N, MOD))