Problem p02632 (C++) — buggy submission, verdict: Runtime Error

// #pragma GCC target("avx,avx2,fma")
// #pragma GCC optimization ("Ofast")
// #pragma GCC optimization ("unroll-loops")
#include <bits/stdc++.h>

using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
// template <typename T>
// using o_set = tree <T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>; #define sfind(i) find_by_order(i) #define
// ord(i) order_of_key(i)
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pii> vii;
typedef vector<pll> vll;
#define mod 1000000007
#define rep(i, a, b) for (ll i = a; i < b; i++)
#define repc(i, a, b, c)                                                       \\
  for (ll i = a; (c > 0 ? i < b : i > b) and c != 0; i += c)
#define vi vector<int>
#define vl vector<ll>
#define _max(vec) (*max_element(all(vec)))
#define _min(vec) (*min_element(all(vec)))
#define fi first
#define si second
#define pb push_back
#define all(name) name.begin(), name.end()
#define ral(name) name.rbegin(), name.rend()
#define fact(n) rep(i, 1, n + 1) ft.pb((ft[i - 1] * i) % mod)
#define mod_in(a) power(a, mod - 2)
#define ncr(n, r)                                                              \\
  (n >= r ? ((ft[n] * mod_in((ft[r] * ft[(n) - (r)]) % mod)) % mod) : 0LL)
#define deb(x) cerr << "[" << #x << " " << x << "]" << endl;
#define endl '\\n' //....????
#define int ll
vl ft(1, 1LL);

/*======================================================================*/
ll power(ll x, ll y) {
  ll res = 1;
  x %= mod;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % mod;
    y = y >> 1;
    x = (x * x) % mod;
  }
  return res % mod;
}
void solve() {
  fact(2000005);
  int k;
  string s;
  cin >> k >> s;
  int ans = 0, n = s.size();
  rep(i, 0, k + 1) {
    ans += (ncr(n + k, i + n) * power(25, k - i)) % mod;
    if (ans >= mod)
      ans -= mod;
  }
  cout << ans;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  // cin >> t;
  while (t--) {
    solve();
  }
}

Error: WA

Fixed submission:

// #pragma GCC target("avx,avx2,fma")
// #pragma GCC optimization ("Ofast")
// #pragma GCC optimization ("unroll-loops")
#include <bits/stdc++.h>

using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
// template <typename T>
// using o_set = tree <T, null_type, less<T>, rb_tree_tag,
// tree_order_statistics_node_update>; #define sfind(i) find_by_order(i) #define
// ord(i) order_of_key(i)
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pii> vii;
typedef vector<pll> vll;
#define mod 1000000007
#define rep(i, a, b) for (ll i = a; i < b; i++)
#define repc(i, a, b, c)                                                       \\
  for (ll i = a; (c > 0 ? i < b : i > b) and c != 0; i += c)
#define vi vector<int>
#define vl vector<ll>
#define _max(vec) (*max_element(all(vec)))
#define _min(vec) (*min_element(all(vec)))
#define fi first
#define si second
#define pb push_back
#define all(name) name.begin(), name.end()
#define ral(name) name.rbegin(), name.rend()
#define fact(n) rep(i, 1, n + 1) ft.pb((ft[i - 1] * i) % mod)
#define mod_in(a) power(a, mod - 2)
#define ncr(n, r)                                                              \\
  (n >= r ? ((ft[n] * mod_in((ft[r] * ft[(n) - (r)]) % mod)) % mod) : 0LL)
#define deb(x) cerr << "[" << #x << " " << x << "]" << endl;
#define endl '\\n' //....????
#define int ll
vl ft(1, 1LL);

/*======================================================================*/
ll power(ll x, ll y) {
  ll res = 1;
  x %= mod;
  while (y > 0) {
    if (y & 1)
      res = (res * x) % mod;
    y = y >> 1;
    x = (x * x) % mod;
  }
  return res % mod;
}
void solve() {
  fact(1000005);
  int k;
  string s;
  cin >> k >> s;
  int ans = 0, n = s.size();
  rep(i, 0, k + 1) {
    ans += (ncr(n + k, i + n) * power(25, k - i)) % mod;
    if (ans >= mod)
      ans -= mod;
  }
  cout << ans;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  // cin >> t;
  while (t--) {
    solve();
  }
}