Buggy python submission (verdict: Runtime Error):
import sys

stdin = sys.stdin


def ni(): return int(ns())


def na(): return list(map(int, stdin.readline().split()))


def naa(N): return [na() for _ in range(N)]


def ns(): return stdin.readline().rstrip()  # ignore trailing spaces

# 大きい数用


mod = 10**9 + 7
N = 10 ** 6  # 出力の制限
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    inverse.append((-inverse[mod % i] * (mod // i)) % mod)


K = ni()
S = ns()
s_len = len(S)
mod = 10 ** 9 + 7
ans = 0

total = K + s_len

rui = 1

cmb = 1

for i in range(total, s_len - 1, -1):
    ans = (ans + cmb * rui) % mod
    # print(ans)
    rui = (rui * 25) % mod
    cmb = (cmb * i * inverse[total - i + 1]) % mod
print(ans)



Fixed submission:
import sys

stdin = sys.stdin


def ni(): return int(ns())


def na(): return list(map(int, stdin.readline().split()))


def naa(N): return [na() for _ in range(N)]


def ns(): return stdin.readline().rstrip()  # ignore trailing spaces

# 大きい数用


mod = 10**9 + 7
N = 10 ** 6 + 1  # 出力の制限
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    inverse.append((-inverse[mod % i] * (mod // i)) % mod)

K = ni()
S = ns()
s_len = len(S)
mod = 10 ** 9 + 7
ans = 0

total = K + s_len

rui = 1

cmb = 1

for i in range(total, s_len - 1, -1):
    # print(total - i + 1)
    ans = (ans + cmb * rui) % mod
    # print(ans)
    rui = (rui * 25) % mod
    cmb = (cmb * i * inverse[total - i + 1]) % mod
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys

stdin = sys.stdin


def ni(): return int(ns())


def na(): return list(map(int, stdin.readline().split()))


def naa(N): return [na() for _ in range(N)]


def ns(): return stdin.readline().rstrip()  # ignore trailing spaces

# 大きい数用


def modPow(a, n, p):
    if n == 0:
        return 1
    if n == 1:
        return a
    if n % 2 == 1:
        return (a * (modPow(a, n // 2, p) ** 2)) % p
    else:
        return (modPow(a, n // 2, p) ** 2) % p


def modInverse(a, p):
    # calculates the modular multiplicative of a mod m.
    # (assuming p is prime).
    return modPow(a, p - 2, p)


def modBinomial(n, k, p):
    # calculates C(n,k) mod p (assuming p is prime).

    numerator = 1  # n * (n-1) * ... * (n-k+1)
    for i in range(k):
        numerator = (numerator * (n - i)) % p

    denominator = 1  # k!
    for i in range(1, k + 1):
        denominator = (denominator * i) % p

    # numerator / denominator mod p.
    return (numerator * modInverse(denominator, p)) % p


K = ni()
S = ns()
s_len = len(S)
mod = 10 ** 9 + 7
ans = 0

total = K + s_len

rui = 1

cmb = 1

for i in range(total, s_len - 1, -1):
    ans = (ans + cmb * rui) % mod
    # print(ans)
    rui = (rui * 25) % mod
    cmb = (cmb * i * modInverse(total - i + 1, mod)) % mod
print(ans)


# a_array = [[0] * 9 for _ in range(9)]
# a_array[0][0] = 1
# for i in range(1,9):
#     for j in range(9):
#         if j == 0:
#             a_array[i][j] = a_array[i-1][j] * 25
#         else:
#             a_array[i][j] = a_array[i-1][j] * 25 + a_array[i-1][j-1]

# print(sum(a_array[-1][3:]) % (10 ** 9 + 7))


Fixed submission:
import sys

stdin = sys.stdin


def ni(): return int(ns())


def na(): return list(map(int, stdin.readline().split()))


def naa(N): return [na() for _ in range(N)]


def ns(): return stdin.readline().rstrip()  # ignore trailing spaces

# 大きい数用


mod = 10**9 + 7
N = 10 ** 6 + 1  # 出力の制限
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    inverse.append((-inverse[mod % i] * (mod // i)) % mod)

K = ni()
S = ns()
s_len = len(S)
mod = 10 ** 9 + 7
ans = 0

total = K + s_len

rui = 1

cmb = 1

for i in range(total, s_len - 1, -1):
    # print(total - i + 1)
    ans = (ans + cmb * rui) % mod
    # print(ans)
    rui = (rui * 25) % mod
    cmb = (cmb * i * inverse[total - i + 1]) % mod
print(ans)