Problem p03861 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

/* #pragma */
/*
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
*/

/*alias*/
#define F first
#define S second

#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev
#define itn int

/*template*/
template <class T> inline bool amax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool amin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

/* func */
int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
long long GCD(long long a, long long b) { return b ? GCD(b, a % b) : a; }
int LCM(int a, int b) { return a * b / GCD(a, b); }
long long LCM(long long a, long long b) { return a * b / GCD(a, b); }

/*struct*/
struct aaa {
  aaa() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} aaaaaaa;

/* const */
const int INF = 1001001001;
const long long LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7; // 10^9
const double EPS = 1e-9;
const int dx[] = {1, 1, 0, -1, -1, -1, 0, 1},
          dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
// const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
// Counterclockwise from the 6o'clock direction
// Take the x-axis downward and the y-axis right

// #define int long long

// g++ -std=c++14 -Wall --pedantic-errors -o template template.cpp
// chcp 65001

signed main() {
  int a, b, x;
  cin >> a >> b >> x;

  if (a == 0) {
    cout << b / x + 1 << endl;
  } else {
    cout << b / x - (a - 1) / x << endl;
  }
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;

/* #pragma */
/*
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
*/

/*alias*/
#define F first
#define S second

#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev
#define itn int

/*template*/
template <class T> inline bool amax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool amin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

/* func */
int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
long long GCD(long long a, long long b) { return b ? GCD(b, a % b) : a; }
int LCM(int a, int b) { return a * b / GCD(a, b); }
long long LCM(long long a, long long b) { return a * b / GCD(a, b); }

/*struct*/
struct aaa {
  aaa() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} aaaaaaa;

/* const */
const int INF = 1001001001;
const long long LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7; // 10^9
const double EPS = 1e-9;
const int dx[] = {1, 1, 0, -1, -1, -1, 0, 1},
          dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
// const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
// Counterclockwise from the 6o'clock direction
// Take the x-axis downward and the y-axis right

#define int long long

// g++ -std=c++14 -Wall --pedantic-errors -o template template.cpp
// chcp 65001

signed main() {
  int a, b, x;
  cin >> a >> b >> x;

  if (a == 0) {
    cout << b / x + 1 << endl;
  } else {
    cout << b / x - (a - 1) / x << endl;
  }
  return 0;
}