Buggy python submission (verdict: Runtime Error):
K = int(input())
S = input()
MOD = 10**9 + 7


def cmb(n, r, MOD):
    if (r < 0 or r > n):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % MOD


N = 10**6+1
g1 = [1, 1]  # 元テーブル
g2 = [1, 1]  # 逆元テーブル
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    g1.append((g1[-1] * i) % MOD)
    inverse.append((-inverse[MOD % i] * (MOD//i)) % MOD)
    g2.append((g2[-1] * inverse[-1]) % MOD)

ans = 0
for i in range(len(S), len(S)+K+1):
    ans += cmb(i-1, len(S)-1, MOD)*pow(25, i-len(S), MOD) * \
        pow(26, len(S)+K-i, MOD)
    ans %= MOD
print(ans)


Fixed submission:
K = int(input())
S = input()
MOD = 10**9 + 7


def cmb(n, r, MOD):
    if (r < 0 or r > n):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % MOD


N = 3*10**6
g1 = [1, 1]  # 元テーブル
g2 = [1, 1]  # 逆元テーブル
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    g1.append((g1[-1] * i) % MOD)
    inverse.append((-inverse[MOD % i] * (MOD//i)) % MOD)
    g2.append((g2[-1] * inverse[-1]) % MOD)

ans = 0
for i in range(len(S), len(S)+K+1):
    ans += cmb(i-1, len(S)-1, MOD)*pow(25, i-len(S), MOD) * \
        pow(26, len(S)+K-i, MOD)
    ans %= MOD
print(ans)

Buggy python submission (verdict: Runtime Error):
K = int(input())
S = input()
MOD = 10**9 + 7


def cmb(n, r, MOD):
    if (r < 0 or r > n):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % MOD


N = 10**6+1
g1 = [1, 1]  # 元テーブル
g2 = [1, 1]  # 逆元テーブル
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    g1.append((g1[-1] * i) % MOD)
    inverse.append((-inverse[MOD % i] * (MOD//i)) % MOD)
    g2.append((g2[-1] * inverse[-1]) % MOD)

ans = 0
for i in range(len(S), len(S)+K+1):
    ans += cmb(i-1, len(S)-1, MOD)*pow(25, i-len(S), MOD) * \
        pow(26, len(S)+K-i, MOD)
    ans %= MOD
print(ans)


Fixed submission:
K = int(input())
S = input()
MOD = 10**9 + 7


def cmb(n, r, MOD):
    if (r < 0 or r > n):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % MOD


N = 3*10**6
g1 = [1, 1]  # 元テーブル
g2 = [1, 1]  # 逆元テーブル
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    g1.append((g1[-1] * i) % MOD)
    inverse.append((-inverse[MOD % i] * (MOD//i)) % MOD)
    g2.append((g2[-1] * inverse[-1]) % MOD)

ans = 0
for i in range(len(S), len(S)+K+1):
    ans += cmb(i-1, len(S)-1, MOD)*pow(25, i-len(S), MOD) * \
        pow(26, len(S)+K-i, MOD)
    ans %= MOD
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
K = int(input())
S = input()
MOD = 10**9 + 7

dp = [[0 for j in range(len(S)+1)] for i in range(len(S)+K+1)]
dp[0][0] = 1

for i in range(len(S)+K):
    for j in range(len(S)+1):
        if j == len(S):
            dp[i+1][j] += dp[i][j]*26
            dp[i+1][j] %= MOD
        else:
            dp[i+1][j] += dp[i][j]*25
            dp[i+1][j] %= MOD
            dp[i+1][j+1] += dp[i][j]
            dp[i+1][j+1] %= MOD

print(dp[len(S)+K][len(S)])


Fixed submission:
K = int(input())
S = input()
MOD = 10**9 + 7


def cmb(n, r, MOD):
    if (r < 0 or r > n):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % MOD


N = 3*10**6
g1 = [1, 1]  # 元テーブル
g2 = [1, 1]  # 逆元テーブル
inverse = [0, 1]  # 逆元テーブル計算用テーブル

for i in range(2, N + 1):
    g1.append((g1[-1] * i) % MOD)
    inverse.append((-inverse[MOD % i] * (MOD//i)) % MOD)
    g2.append((g2[-1] * inverse[-1]) % MOD)

ans = 0
for i in range(len(S), len(S)+K+1):
    ans += cmb(i-1, len(S)-1, MOD)*pow(25, i-len(S), MOD) * \
        pow(26, len(S)+K-i, MOD)
    ans %= MOD
print(ans)