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

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> //required
#include <ext/pb_ds/tree_policy.hpp>     //required
#define pb push_back
#define pf push_front
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) (long long)(x).size()
#define sqr(x) (x) * (x)
#define sof sizeof
#define resz resize
#define ins insert
#define lowerB lower_bound
#define upperB upper_bound
#define mkp make_pair
#define mkt make_tuple
#define act(a, v) (get<(long long)a>(v))
#define mod 1000000007
#define mod1 998244353
#define N 2000003 // N idhar hai
#define inf 1e18
#define minf -1e18
#define min3(a, b, c) min(a, min(b, c));
#define max3(a, b, c) max(a, max(b, c));
#define Foreach(i, c)                                                          \\
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define For(i, a, b) for (int(i) = (a); (i) < (b); ++(i))
#define rof(i, a, b) for (int(i) = (a); (i) >= (b); --(i))
#define rep(i, c) for (auto &(i) : (c))
#define sumall(v) accumulate(all(v), (ll)0)
#define suma accumulate
#define FastIO                                                                 \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL)
#define PRECISION(n)                                                           \\
  std::cout.unsetf(std::ios::floatfield);                                      \\
  std::cout.precision((ll)n)
#define getmx(v) max_element(all(v))
#define getmn(v) min_element(all(v))
#define wr1(v) cout << v << endl
#define wr2(a, b) cout << a << " " << b << endl
#define wr3(a, b, c) cout << a << " " << b << " " << c << endl
#define FileIO                                                                 \\
  freopen("input.txt", "r", stdin);                                            \\
  freopen("output.txt", "w", stdout)
#define call(dr)                                                               \\
  dr.pb(mkp(1, 0));                                                            \\
  dr.pb(mkp(-1, 0));                                                           \\
  dr.pb(mkp(0, 1));                                                            \\
  dr.pb(mkp(0, -1))
#define fill(dp, a, b)                                                         \\
  for (int i = 0; i < a; i++)                                                  \\
    for (int j = 0; j < b; j++)                                                \\
      dp[i][j] = -1;
#define PI M_PI
#define fi first
#define se second
#define here cout << "RSSB" << endl
#define showzero cout << std::showpoint
#define bitcount(x) __builtin_popcountll(x)
#define ls(i) (1ll << (i))
#define rs(i) ((i) >> 1)
#define br "\\n"
#define sp " "
using namespace __gnu_pbds; // required
using namespace std;
typedef double db;
typedef long long ll;
typedef vector<int> vi;
// typedef vector<l>vl ;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<string> vstr;
typedef vector<bool> vbool;
template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
void READ(vll &v, ll n) {
  ll a;
  for (ll i = 0; i < n; i = i + 1) {
    cin >> a;
    v.pb(a);
  }
}
void PRINT(vll &v, ll a = 0) {
  for (ll i = a; i < v.size(); i = i + 1) { /*v[i] = max((ll)0,v[i]);*/
    cout << v[i] << " ";
  }
  cout << "\\n";
}
double logy(ll n, ll b) {
  if (b == 0) {
    return (-1);
  }
  if (n == 1) {
    return (0);
  }
  return ((db)log10(n) / log10(b));
}
long long power(long long k, long long n, long long m = mod) {
  long long res = 1;
  while (n) {
    if (n % 2 != 0) {
      res = (res * k) % m;
    }
    k = (k * k) % m;
    n = n / 2;
  }
  return (res);
}
double power(double k, long long n) {
  double res = 1;
  while (n) {
    if (n % 2 != 0) {
      res = (res * k);
    }
    k = (k * k);
    n = n / 2;
  }
  return (res);
}
ll sign(ll i) {
  if (i == 0) {
    return (0);
  }
  if (i > 0) {
    return (1);
  }
  return (-1);
}
double distance(pll a, pll b) {
  return ((db)(sqrt(sqr(a.fi - b.fi) + sqr(a.se - b.se))));
}
double distance(pair<ll, pll> a, pair<ll, pll> b) {
  return ((db)(sqrt(sqr(a.fi - b.fi) + sqr(a.se.fi - b.se.fi) +
                    sqr(a.se.se - b.se.se))));
}
ll lis(vll v) {
  ll d = sz(v);
  vll dis(d + 1, inf);
  for (ll i = 0; i < d; i++) {
    *lowerB(all(dis), v[i]) = v[i];
  }
  for (ll i = 0; i < d + 1; i++) {
    if (dis[i] == inf) {
      return (i);
    }
  }
  return (1);
}
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
#define lcm(a, b) ((a / gcd(a, b)) * b)
const ll dx[8] = {-1, -1, -1, 0, 1, 1, 1, 0};
const ll dy[8] = {-1, 0, 1, 1, 1, 0, -1, -1};
vpll dr;
ll n, m, k, h, a1, a2, t;
db prob;
string str;
bool ok = true;
int main() {
  FastIO;
  PRECISION(10);
  ll a, b, c, d, i, j, x, y, z; //,x1,y1,z1,e,f;
  ll p, q, r, u, w;
  char ch, chr;
  string str1, str2, str3, str4;
  // ll a1=0,a2=0,b1=0,b2=0,c1=0 ;
  // cin>>str>>str1;
  // Listen to Zero(Imagine Dragons!!)
  // Stay Motivated!!
  // Never use i for two or more nested loops!!
  // Range Query + Constructive + DSU +  Graph Theory + Math + Number Theory +
  // DP + Bitmasks + Data Structures + Strings + Geometry + Brute Force + Greedy
  // + Implementation Binary and Ternary Search In a comparator function
  // comp(pll a , pll b) if comp returns true then a comes before b otherwise b
  // comes berfore a Fuck Excuses!!Focus on how to get better call(dr); 18
  cin >> k >> str;
  n = sz(str);
  vll fact(N, 1);
  for (i = 2; i < N; i++) {
    fact[i] = (fact[i - 1] % mod * i % mod) % mod;
  }
  x = 0;
  z = fact[n + k];
  for (i = n; i <= n + k; i++) {
    // i characters necessary for initial string
    y = (fact[i] % mod * fact[n + k - i] % mod) % mod;
    y = power(y, mod - 2, mod) % mod;
    y = (z % mod * y % mod) % mod;
    p = power(25, n + k - i, mod);
    x = (x % mod + (y % mod * p % mod) % mod) % mod;
  }
  cout << x;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> //required
#include <ext/pb_ds/tree_policy.hpp>     //required
#define pb push_back
#define pf push_front
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) (long long)(x).size()
#define sqr(x) (x) * (x)
#define sof sizeof
#define resz resize
#define ins insert
#define lowerB lower_bound
#define upperB upper_bound
#define mkp make_pair
#define mkt make_tuple
#define act(a, v) (get<(long long)a>(v))
#define mod 1000000007
#define mod1 998244353
#define N 800003 // N idhar hai
#define inf 1e18
#define minf -1e18
#define min3(a, b, c) min(a, min(b, c));
#define max3(a, b, c) max(a, max(b, c));
#define Foreach(i, c)                                                          \\
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define For(i, a, b) for (int(i) = (a); (i) < (b); ++(i))
#define rof(i, a, b) for (int(i) = (a); (i) >= (b); --(i))
#define rep(i, c) for (auto &(i) : (c))
#define sumall(v) accumulate(all(v), (ll)0)
#define suma accumulate
#define FastIO                                                                 \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(NULL);                                                               \\
  cout.tie(NULL)
#define PRECISION(n)                                                           \\
  std::cout.unsetf(std::ios::floatfield);                                      \\
  std::cout.precision((ll)n)
#define getmx(v) max_element(all(v))
#define getmn(v) min_element(all(v))
#define wr1(v) cout << v << endl
#define wr2(a, b) cout << a << " " << b << endl
#define wr3(a, b, c) cout << a << " " << b << " " << c << endl
#define FileIO                                                                 \\
  freopen("input.txt", "r", stdin);                                            \\
  freopen("output.txt", "w", stdout)
#define call(dr)                                                               \\
  dr.pb(mkp(1, 0));                                                            \\
  dr.pb(mkp(-1, 0));                                                           \\
  dr.pb(mkp(0, 1));                                                            \\
  dr.pb(mkp(0, -1))
#define fill(dp, a, b)                                                         \\
  for (int i = 0; i < a; i++)                                                  \\
    for (int j = 0; j < b; j++)                                                \\
      dp[i][j] = -1;
#define PI M_PI
#define fi first
#define se second
#define here cout << "RSSB" << endl
#define showzero cout << std::showpoint
#define bitcount(x) __builtin_popcountll(x)
#define ls(i) (1ll << (i))
#define rs(i) ((i) >> 1)
#define br "\\n"
#define sp " "
using namespace __gnu_pbds; // required
using namespace std;
typedef double db;
typedef long long ll;
typedef vector<int> vi;
// typedef vector<l>vl ;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<string> vstr;
typedef vector<bool> vbool;
template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
void READ(vll &v, ll n) {
  ll a;
  for (ll i = 0; i < n; i = i + 1) {
    cin >> a;
    v.pb(a);
  }
}
void PRINT(vll &v, ll a = 0) {
  for (ll i = a; i < v.size(); i = i + 1) { /*v[i] = max((ll)0,v[i]);*/
    cout << v[i] << " ";
  }
  cout << "\\n";
}
double logy(ll n, ll b) {
  if (b == 0) {
    return (-1);
  }
  if (n == 1) {
    return (0);
  }
  return ((db)log10(n) / log10(b));
}
long long power(long long k, long long n, long long m = mod) {
  long long res = 1;
  while (n) {
    if (n % 2 != 0) {
      res = (res * k) % m;
    }
    k = (k * k) % m;
    n = n / 2;
  }
  return (res);
}
double power(double k, long long n) {
  double res = 1;
  while (n) {
    if (n % 2 != 0) {
      res = (res * k);
    }
    k = (k * k);
    n = n / 2;
  }
  return (res);
}
ll sign(ll i) {
  if (i == 0) {
    return (0);
  }
  if (i > 0) {
    return (1);
  }
  return (-1);
}
double distance(pll a, pll b) {
  return ((db)(sqrt(sqr(a.fi - b.fi) + sqr(a.se - b.se))));
}
double distance(pair<ll, pll> a, pair<ll, pll> b) {
  return ((db)(sqrt(sqr(a.fi - b.fi) + sqr(a.se.fi - b.se.fi) +
                    sqr(a.se.se - b.se.se))));
}
ll lis(vll v) {
  ll d = sz(v);
  vll dis(d + 1, inf);
  for (ll i = 0; i < d; i++) {
    *lowerB(all(dis), v[i]) = v[i];
  }
  for (ll i = 0; i < d + 1; i++) {
    if (dis[i] == inf) {
      return (i);
    }
  }
  return (1);
}
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
#define lcm(a, b) ((a / gcd(a, b)) * b)
const ll dx[8] = {-1, -1, -1, 0, 1, 1, 1, 0};
const ll dy[8] = {-1, 0, 1, 1, 1, 0, -1, -1};
vpll dr;
ll n, m, k, h, a1, a2, t;
db prob;
string str;
bool ok = true;
int main() {
  FastIO;
  PRECISION(10);
  ll a, b, c, d, i, j, x, y, z; //,x1,y1,z1,e,f;
  ll p, q, r, u, w;
  char ch, chr;
  string str1, str2, str3, str4;
  // ll a1=0,a2=0,b1=0,b2=0,c1=0 ;
  // cin>>str>>str1;
  // Listen to Zero(Imagine Dragons!!)
  // Stay Motivated!!
  // Never use i for two or more nested loops!!
  // Range Query + Constructive + DSU +  Graph Theory + Math + Number Theory +
  // DP + Bitmasks + Data Structures + Strings + Geometry + Brute Force + Greedy
  // + Implementation Binary and Ternary Search In a comparator function
  // comp(pll a , pll b) if comp returns true then a comes before b otherwise b
  // comes berfore a Fuck Excuses!!Focus on how to get better call(dr); 18
  cin >> k >> str;
  n = sz(str);
  vll fact(N, 1);
  for (i = 2; i < N; i++) {
    fact[i] = (fact[i - 1] % mod * i % mod) % mod;
  }
  x = 0;
  z = fact[n + k];
  for (i = n; i <= n + k; i++) {
    // i characters necessary for initial string
    y = (fact[i] % mod * fact[n + k - i] % mod) % mod;
    y = power(y, mod - 2, mod) % mod;
    y = (z % mod * y % mod) % mod;
    p = power(25, n + k - i, mod);
    x = (x % mod + (y % mod * p % mod) % mod) % mod;
  }
  cout << x;
}