Buggy python submission (verdict: Time Limit Exceeded):
mod=10**9+7
k=int(input())
s=input()
l=len(s)
fact=[1]
for i in range(1,k+l+1):
  fact.append((fact[-1]*i)%mod)
revfact=[]
for i in range(k+l+1):
  revfact.append(pow(fact[i],mod-2,mod))
pow1=[1]
pow2=[1]
for i in range(1,k+l+1):
  pow1.append((pow1[-1]*25)%mod)
  pow2.append((pow2[-1]*26)%mod)
ans=0
for i in range(k+l):
  coef1=(pow1[k-i]*pow2[i])%mod
  if i<=k:
    coef2=(fact[k+l-1-i]*revfact[l-1]*revfact[k-i])%mod
  else:
    coef2=0
  ans+=coef1*coef2
  ans%=mod
print(ans)


Fixed submission:
import collections
import heapq

from collections import defaultdict
from heapq import heappush, heappop
import sys


def main(K, N):
    P = 10 ** 9 + 7

    P25 = [1] * (K + 1)
    P26 = [1] * (K + 1)
    F = [1] * (N + K)
    INV = [1] * (N + K)
    INVF = [1] * (N + K)

    p25 = 1
    p26 = 1
    for i in range(1, K + 1):
        p25 *= 25
        p25 %= P
        P25[i] = p25

        p26 *= 26
        p26 %= P
        P26[i] = p26

    f = 1
    invf = 1
    for i in range(2, N + K):
        f *= i
        f %= P
        F[i] = f
        q, r = divmod(P, i)
        INV[i] = -INV[r] * q % P
        invf *= INV[i]
        invf %= P
        INVF[i] = invf

    def comb_rep(n, r):
        return (F[n + r - 1] * INVF[r] % P) * INVF[n - 1] % P

    ret = 0
    for i in range(K + 1):
        ret += (
            P25[i] *
            P26[K - i] % P *
            comb_rep(N, i) % P
        )
    return (ret % P)


if sys.argv[-1] == 'ONLINE_JUDGE':
    print("compiling")
    from numba.pycc import CC
    cc = CC('my_module')
    cc.export('main', 'i8(i8,i8)')(main)
    cc.compile()
    exit()
else:
    from my_module import main
    K = int(input())
    S = input()
    print(main(K, len(S)))

Buggy python submission (verdict: Time Limit Exceeded):
mod=10**9+7
k=int(input())
s=input()
l=len(s)
fact=[1]
for i in range(1,k+l+1):
  fact.append((fact[-1]*i)%mod)
revfact=[]
for i in range(k+l+1):
  revfact.append(pow(fact[i],mod-2,mod))
pow1=[1]
pow2=[1]
for i in range(1,k+l+1):
  pow1.append((pow1[-1]*25)%mod)
  pow2.append((pow2[-1]*26)%mod)
ans=0
for i in range(k+l):
  coef1=(pow1[k-i]*pow2[i])%mod
  if i<=k:
    coef2=(fact[k+l-1-i]*revfact[l-1]*revfact[k-i])%mod
  else:
    coef2=0
  ans+=coef1*coef2
  ans%=mod
print(ans)


Fixed submission:
import collections
import heapq

from collections import defaultdict
from heapq import heappush, heappop
import sys


def main(K, N):
    P = 10 ** 9 + 7

    P25 = [1] * (K + 1)
    P26 = [1] * (K + 1)
    F = [1] * (N + K)
    INV = [1] * (N + K)
    INVF = [1] * (N + K)

    p25 = 1
    p26 = 1
    for i in range(1, K + 1):
        p25 *= 25
        p25 %= P
        P25[i] = p25

        p26 *= 26
        p26 %= P
        P26[i] = p26

    f = 1
    invf = 1
    for i in range(2, N + K):
        f *= i
        f %= P
        F[i] = f
        q, r = divmod(P, i)
        INV[i] = -INV[r] * q % P
        invf *= INV[i]
        invf %= P
        INVF[i] = invf

    def comb_rep(n, r):
        return (F[n + r - 1] * INVF[r] % P) * INVF[n - 1] % P

    ret = 0
    for i in range(K + 1):
        ret += (
            P25[i] *
            P26[K - i] % P *
            comb_rep(N, i) % P
        )
    return (ret % P)


if sys.argv[-1] == 'ONLINE_JUDGE':
    print("compiling")
    from numba.pycc import CC
    cc = CC('my_module')
    cc.export('main', 'i8(i8,i8)')(main)
    cc.compile()
    exit()
else:
    from my_module import main
    K = int(input())
    S = input()
    print(main(K, len(S)))

Buggy python submission (verdict: Time Limit Exceeded):
mod=10**9+7
k=int(input())
s=input()
l=len(s)
fact=[1] #0!～(|S|+k)!の前計算を行う
for i in range(1,k+l+1):
 fact.append((fact[-1]*i)%mod)
revfact=[]
for i in range(k+l+1): #0!～(|S|+k)!の逆数の前計算を行う
 revfact.append(pow(fact[i],mod-2,mod))
pow1=[1]
pow2=[1]
for i in range(1,k+l+1): #25^Kと26^Kの前計算を行う
 pow1.append((pow1[-1]*25)%mod)
 pow2.append((pow2[-1]*26)%mod)
ans=0
for i in range(k+l): #Sの最後の文字を置く場所を|S|+Kマスの中からすべて試す
 coef1=(pow1[k-i]*pow2[i])%mod
 if i<=k:
   coef2=(fact[k+l-1-i]*revfact[l-1]*revfact[k-i])%mod
 else:
   coef2=0
 ans+=coef1*coef2
 ans%=mod
print(ans)


Fixed submission:
import collections
import heapq

from collections import defaultdict
from heapq import heappush, heappop
import sys


def main(K, N):
    P = 10 ** 9 + 7

    P25 = [1] * (K + 1)
    P26 = [1] * (K + 1)
    F = [1] * (N + K)
    INV = [1] * (N + K)
    INVF = [1] * (N + K)

    p25 = 1
    p26 = 1
    for i in range(1, K + 1):
        p25 *= 25
        p25 %= P
        P25[i] = p25

        p26 *= 26
        p26 %= P
        P26[i] = p26

    f = 1
    invf = 1
    for i in range(2, N + K):
        f *= i
        f %= P
        F[i] = f
        q, r = divmod(P, i)
        INV[i] = -INV[r] * q % P
        invf *= INV[i]
        invf %= P
        INVF[i] = invf

    def comb_rep(n, r):
        return (F[n + r - 1] * INVF[r] % P) * INVF[n - 1] % P

    ret = 0
    for i in range(K + 1):
        ret += (
            P25[i] *
            P26[K - i] % P *
            comb_rep(N, i) % P
        )
    return (ret % P)


if sys.argv[-1] == 'ONLINE_JUDGE':
    print("compiling")
    from numba.pycc import CC
    cc = CC('my_module')
    cc.export('main', 'i8(i8,i8)')(main)
    cc.compile()
    exit()
else:
    from my_module import main
    K = int(input())
    S = input()
    print(main(K, len(S)))