Buggy python submission (verdict: Time Limit Exceeded):
mod = 1000000007
fact = []
fact_inv = []
pow_ = []
pow25_ = []
pow26_ = []

def pow_ini(nn):
	global pow_
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return

def pow_ini25(nn):
	global pow25_
	pow25_.append(nn)
	for j in range(62):
		nxt = pow25_[j] * pow25_[j]
		nxt %= mod
		pow25_.append(nxt)
	return

def pow_ini26(nn):
	global pow26_
	pow26_.append(nn)
	for j in range(62):
		nxt = pow26_[j] * pow26_[j]
		nxt %= mod
		pow26_.append(nxt)
	return

def pow1(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow_ini25(25)
pow_ini26(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Fixed submission:
mod = 1000000007
fact = []
fact_inv = []
pow_ = []
pow25_ = []
pow26_ = []

def pow_ini(nn):
	global pow_
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return

def pow_ini25(nn):
	global pow25_
	pow25_.append(nn)
	for j in range(62):
		nxt = pow25_[j] * pow25_[j]
		nxt %= mod
		pow25_.append(nxt)
	return

def pow_ini26(nn):
	global pow26_
	pow26_.append(nn)
	for j in range(62):
		nxt = pow26_[j] * pow26_[j]
		nxt %= mod
		pow26_.append(nxt)
	return

def pow1(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow_ini25(25)
pow_ini26(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
mod = 1000000007
fact = []
fact_inv = []
pow_ = []

def pow_ini(nn):
	pow_ = []
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return pow_

def pow1(k):
	global pow_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	global pow25_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	global pow26_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ = pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow25_ = pow_ini(25)
pow26_ = pow_ini(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Fixed submission:
mod = 1000000007
fact = []
fact_inv = []
pow_ = []
pow25_ = []
pow26_ = []

def pow_ini(nn):
	global pow_
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return

def pow_ini25(nn):
	global pow25_
	pow25_.append(nn)
	for j in range(62):
		nxt = pow25_[j] * pow25_[j]
		nxt %= mod
		pow25_.append(nxt)
	return

def pow_ini26(nn):
	global pow26_
	pow26_.append(nn)
	for j in range(62):
		nxt = pow26_[j] * pow26_[j]
		nxt %= mod
		pow26_.append(nxt)
	return

def pow1(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow_ini25(25)
pow_ini26(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Buggy python submission (verdict: Runtime Error):
mod = 1000000007
fact = []
fact_inv = []
pow_ = []

def pow_ini(nn):
	pow_ = []
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return pow_

def pow1(k):
	global pow_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	global pow25_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	global pow26_
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ = pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(1100000)
pow25_ = pow_ini(25)
pow26_ = pow_ini(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Fixed submission:
mod = 1000000007
fact = []
fact_inv = []
pow_ = []
pow25_ = []
pow26_ = []

def pow_ini(nn):
	global pow_
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return

def pow_ini25(nn):
	global pow25_
	pow25_.append(nn)
	for j in range(62):
		nxt = pow25_[j] * pow25_[j]
		nxt %= mod
		pow25_.append(nxt)
	return

def pow_ini26(nn):
	global pow26_
	pow26_.append(nn)
	for j in range(62):
		nxt = pow26_[j] * pow26_[j]
		nxt %= mod
		pow26_.append(nxt)
	return

def pow1(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow_ini25(25)
pow_ini26(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)

Buggy python submission (verdict: Runtime Error):
#include <iostream>
#include <string>
#include <algorithm>
#include <cmath>
#include <functional>
#include <vector>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <cassert>
#define InfL 1000000000
#define InfLL 1000000000000000000LL
#define nmax_def 110000
#define Cmax_def 2200
#define mod 1000000007
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=(n-1);(i)>=(0);(i)--)
using namespace std;
typedef long long ll;
typedef double db;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<bool> vb;
typedef vector<db> vd;


vl pow_;
vl pow_1;
vl pow_2;
void pow_ini(ll n) {
	pow_.clear();
	pow_.push_back(n);
	rep(j, 62) {
		ll nxt = pow_[j] * pow_[j];
		nxt %= mod;
		pow_.push_back(nxt);
	}
	return;
}
void pow_ini1(ll n) {
	pow_1.clear();
	pow_1.push_back(n);
	rep(j, 62) {
		ll nxt = pow_1[j] * pow_1[j];
		nxt %= mod;
		pow_1.push_back(nxt);
	}
	return;
}
void pow_ini2(ll n) {
	pow_2.clear();
	pow_2.push_back(n);
	rep(j, 62) {
		ll nxt = pow_2[j] * pow_2[j];
		nxt %= mod;
		pow_2.push_back(nxt);
	}
	return;
}

ll pow1(ll k) {
	ll ans = 1;
	rep(ntmp, 62) {
		if ((k >> ntmp) % 2 == 1) {
			ans *= pow_[ntmp];
			ans %= mod;
		}
	}
	return ans;
}
ll pow25(ll k) {
	ll ans = 1;
	rep(ntmp, 62) {
		if ((k >> ntmp) % 2 == 1) {
			ans *= pow_1[ntmp];
			ans %= mod;
		}
	}
	return ans;
}
ll pow26(ll k) {
	ll ans = 1;
	rep(ntmp, 62) {
		if ((k >> ntmp) % 2 == 1) {
			ans *= pow_2[ntmp];
			ans %= mod;
		}
	}
	return ans;
}

vl fact;
vl fact_inv;

void fact_ini(ll n) {
	fact.resize(n + 1);
	fact[0] = 1;
	for (ll i = 1; i <= n; i++) {
		ll fact_tmp = fact[i - 1] * i;
		fact_tmp %= mod;
		fact[i] = fact_tmp;
	}
	fact_inv.resize(n + 1);
	pow_ini(fact[n]);
	fact_inv[n] = pow1(mod - 2);
	for (ll i = n - 1; i >= 0; i--) {
		fact_inv[i] = fact_inv[i + 1] * (i + 1);
		fact_inv[i] %= mod;
	}
}

ll nCm(ll n, ll m) {
	assert(m >= 0);
	assert(n < fact.size());
	assert(n >= m);
	ll ans = fact[n] * fact_inv[m];
	ans %= mod;
	ans *= fact_inv[n - m];
	ans %= mod;
	return ans;
}

int main() {
	fact_ini(2200000);
	pow_ini1(25);
	pow_ini2(26);
	ll K;
	cin >> K;
	string S;
	cin >> S;
	ll N = S.length();
	ll ans = 0;
	for (ll k = 0; k <= K; k++) {
		ll n = N + K - k;
		ll anstmp = 1;
		anstmp *= pow26(k);
		anstmp %= mod;
		anstmp *= nCm(n - 1, N - 1);
		anstmp %= mod;
		anstmp *= pow25(n - N);
		anstmp %= mod;
		ans += anstmp;
		ans %= mod;

	}
	cout << ans << endl;
	return 0;
}

Fixed submission:
mod = 1000000007
fact = []
fact_inv = []
pow_ = []
pow25_ = []
pow26_ = []

def pow_ini(nn):
	global pow_
	pow_.append(nn)
	for j in range(62):
		nxt = pow_[j] * pow_[j]
		nxt %= mod
		pow_.append(nxt)
	return

def pow_ini25(nn):
	global pow25_
	pow25_.append(nn)
	for j in range(62):
		nxt = pow25_[j] * pow25_[j]
		nxt %= mod
		pow25_.append(nxt)
	return

def pow_ini26(nn):
	global pow26_
	pow26_.append(nn)
	for j in range(62):
		nxt = pow26_[j] * pow26_[j]
		nxt %= mod
		pow26_.append(nxt)
	return

def pow1(k):
	ansk = 1
	for ntmp in range(62):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow_[ntmp]
			ansk %= mod
	return ansk

def pow25(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow25_[ntmp]
			ansk %= mod
	return ansk

def pow26(k):
	ansk = 1
	for ntmp in range(31):
		if((k >> ntmp) % 2 == 1):
			ansk *= pow26_[ntmp]
			ansk %= mod
	return ansk

def fact_ini(n):
	global fact
	global fact_inv
	global pow_
	for i in range(n + 1):
		fact.append(0)
		fact_inv.append(0)
	fact[0] = 1
	for i in range(1, n + 1, 1):
		fact_tmp = fact[i - 1] * i
		fact_tmp %= mod
		fact[i] = fact_tmp
	pow_ini(fact[n])
	fact_inv[n] = pow1(mod - 2)
	for i in range(n - 1, -1, -1):
		fact_inv[i] = fact_inv[i + 1] * (i + 1)
		fact_inv[i] %= mod
	return

def nCm(n, m):
	assert(m >= 0)
	assert(n >= m)
	ans = fact[n] * fact_inv[m]
	ans %= mod
	ans *= fact_inv[n - m]
	ans %= mod
	return ans;

fact_ini(2200000)
pow_ini25(25)
pow_ini26(26)
K = int(input())
S = input()
N = len(S)
ans = 0
for k in range(0, K+1, 1):
	n = N + K - k
	anstmp = 1
	anstmp *= pow26(k)
	anstmp %= mod
	anstmp *= nCm(n - 1, N - 1)
	anstmp %= mod
	anstmp *= pow25(n - N)
	anstmp %= mod
	ans += anstmp
	ans %= mod
print(ans)