Buggy python submission (verdict: Runtime Error):
k = int(input())
# s = input()
# s = len(s)
s = int(input())

mod = 10 ** 9 + 7
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[_n] = pow(_fac[_n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n-s):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + fac[i-1] * inv[s-1] * inv[i-s] * n25[i-s] * n26[n-i]) % mod

print(ans)


Fixed submission:
k = int(input())
s = input()
s = len(s)

mod = 10 ** 9 + 7
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[_n] = pow(_fac[_n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n-s):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + fac[i-1] * inv[s-1] * inv[i-s] * n25[i-s] * n26[n-i]) % mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
k = int(input())
s = input()

mod = 10 ** 9 + 7
s = len(s)
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[n] = pow(_fac[n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + fac[i-1] * inv[s-1] * inv[i - s] * n25[i - s] * n26[n - i]) % mod

print(ans)


Fixed submission:
k = int(input())
s = input()
s = len(s)

mod = 10 ** 9 + 7
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[_n] = pow(_fac[_n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n-s):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + fac[i-1] * inv[s-1] * inv[i-s] * n25[i-s] * n26[n-i]) % mod

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
k = int(input())
s = input()

mod = 10 ** 9 + 7
s = len(s)
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[n] = pow(_fac[n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + (fac[i-1] * inv[s-1] * inv[i - s] * n25[i - s]) * n26[n - i]) % mod

print(ans)


Fixed submission:
k = int(input())
s = input()
s = len(s)

mod = 10 ** 9 + 7
n = k + s


def _fac_inv(_n, _mod):
    _fac = [1] * (_n + 1)
    _inv = [1] * (_n + 1)
    for i in range(_n):
        _fac[i + 1] = _fac[i] * (i + 1) % _mod
    _inv[_n] = pow(_fac[_n], _mod - 2, _mod)
    for i in range(_n, 0, -1):
        _inv[i - 1] = _inv[i] * i % _mod

    return _fac, _inv


fac, inv = _fac_inv(n, mod)

n25 = [1]
n26 = [1]
for _ in range(n-s):
    n25.append((n25[-1] * 25) % mod)
    n26.append((n26[-1] * 26) % mod)

ans = 0

for i in range(s, n + 1):
    ans = (ans + fac[i-1] * inv[s-1] * inv[i-s] * n25[i-s] * n26[n-i]) % mod

print(ans)