Buggy python submission (verdict: Runtime Error):
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1] * (2 * 10**6 + 10)
f_inv = [1] * (2 * 10**6 + 10)

def prepare(n, mod):
    for i in range(1, n+1):
        fac[i] = (fac[i-1] * i) % mod
    
    f_inv[n] = pow(fac[n], -1, MOD)
    for i in range(n-1, 0, -1):
        f_inv[i] = (f_inv[i+1] * (i+1)) % MOD

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    inv26 = pow(26, -1, MOD)
    pow26 = pow(26, K, MOD)
    pow25 = 1
    ans = 0
    for i in range(K+1):
        ans += (modcmb(N-1+i, i, MOD) * pow25 * pow26) % MOD
        ans %= MOD

        pow25 *= 25
        pow25 %= MOD
        pow26 *= inv26
        pow26 %= MOD

    print(ans)


if __name__ == "__main__":
    main()


Fixed submission:
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * pow(fac[r], -1, MOD) * pow(fac[n-r], -1, MOD) % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    inv26 = pow(26, -1, MOD)
    exp = pow(26, K, MOD)
    ans = 0
    for i in range(K+1):
        tmp = exp * modcmb(N-1+i, i, MOD)
        tmp %= MOD
        ans += tmp
        ans %= MOD

        exp *= 25 * inv26
        exp %= MOD

    print(ans)


if __name__ == "__main__":
    main()


Fixed submission:
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * pow(fac[r], -1, MOD) * pow(fac[n-r], -1, MOD) % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()


Fixed submission:
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * pow(fac[r], MOD-2, MOD) * pow(fac[n-r], MOD-2, MOD) % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()


Fixed submission:
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()


Fixed submission:
import sys
read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines

MOD = 10**9+7

fac = [1, 1]     # 元テーブル
f_inv = [1, 1]   # 逆元テーブル
inv = [0, 1]     # 逆元テーブル計算用テーブル

def prepare(n, mod):
    for i in range(2, n+1):
        fac.append((fac[-1] * i) % mod)
        inv.append((-inv[mod % i] * (mod//i)) % mod)
        f_inv.append((f_inv[-1] * inv[-1]) % mod)

def modcmb(n, r, mod):
    if n < 0 or r < 0:
        return 0
    if r > n:
        return 0

    return fac[n] * f_inv[r] * f_inv[n-r] % mod


def main():
    K = int(readline())
    S = readline().strip()
    N = len(S)

    prepare(N + K + 10, MOD)

    ans = 0
    for i in range(K+1):
        ans += modcmb(N-1+i, i, MOD) * pow(25, i, MOD) * pow(26, K-i, MOD) % MOD
        ans %= MOD

    print(ans)


if __name__ == "__main__":
    main()