Problem p03107 (C++)

Buggy submission (status: Runtime Error):
#pragma GCC target("avx2")
#pragma GCC optimize("Ofast")
#pragma GCC push_options
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define pb push_back
#define __V vector
#define all(x) x.begin(), x.end()
#define oit ostream_iterator
#define mod 998244353ll
using namespace std;
void doin() {
  cin.tie();
  cout.tie();
  ios::sync_with_stdio(0);
#ifndef ONLINE_JUDGE
//	freopen("input.txt", "r", stdin);
//	freopen("output.txt", "w", stdout);
#endif
}
template <typename X, typename Y>
istream &operator>>(istream &in, pair<X, Y> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T> void getv(T &i) { cin >> i; }
template <typename T, typename... Ns> void getv(vector<T> &v, int n, Ns... ns) {
  v.resize(n);
  for (auto &i : v)
    getv(i, ns...);
}
template <typename T> void getv1(T &i) { cin >> i; }
template <typename T, typename... Ns>
void getv1(vector<T> &v, int n, Ns... ns) {
  v.resize(n + 1);
  for (int i = 1; i <= n; i++)
    getv(v[i], ns...);
}
#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
inline void getch(char &x) {
  while (x = getchar_unlocked(), x < 33) {
    ;
  }
}
inline void getstr(string &str) {
  str.clear();
  char cur;
  while (cur = getchar_unlocked(), cur < 33) {
    ;
  }
  while (cur > 32) {
    str += cur;
    cur = getchar_unlocked();
  }
}
template <typename T> inline bool sc(T &num) {
  bool neg = 0;
  int c;
  num = 0;
  while (c = getchar_unlocked(), c < 33) {
    if (c == EOF)
      return false;
  }
  if (c == '-') {
    neg = 1;
    c = getchar_unlocked();
  }
  for (; c > 47; c = getchar_unlocked())
    num = num * 10 + c - 48;
  if (neg)
    num *= -1;
  return true;
}
typedef unsigned long long ull;
typedef long long ll;
typedef float ld;
typedef ll _I;
typedef pair<_I, _I> pi;
typedef pair<ld, ld> pd;
typedef map<_I, _I> mii;
typedef __V<_I> vi;
typedef __V<char> vc;
typedef __V<ld> vd;
typedef __V<vd> vvd;
typedef __V<pi> vpi;
typedef __V<__V<_I>> vvi;
typedef __V<__V<char>> vvc;
typedef __V<__V<pi>> vvpi;
using AntonTsypko = void;
using arsijo = AntonTsypko;
using god = arsijo;
uniform_real_distribution<double> double_dist(0, 1);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::steady_clock::now().time_since_epoch().count());
int main() {
  doin();
  string s;
  stack<char> a;
  cin >> s;
  int ans = 0;
  for (auto i : s) {
    if (!a.empty() && a.top() != i)
      a.pop(), ans += 2;
    else
      a.push(i);
  }
  cout << ans;

  return 0;
}


Fixed submission:
// #pragma GCC target ("avx2")
#pragma GCC optimize("Ofast")
#pragma GCC push_options
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#define pb push_back
#define __V vector
#define all(x) x.begin(), x.end()
#define oit ostream_iterator
#define mod 998244353ll
using namespace std;
void doin() {
  cin.tie();
  cout.tie();
  ios::sync_with_stdio(0);
#ifndef ONLINE_JUDGE
//	freopen("input.txt", "r", stdin);
//	freopen("output.txt", "w", stdout);
#endif
}
template <typename X, typename Y>
istream &operator>>(istream &in, pair<X, Y> &a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T> void getv(T &i) { cin >> i; }
template <typename T, typename... Ns> void getv(vector<T> &v, int n, Ns... ns) {
  v.resize(n);
  for (auto &i : v)
    getv(i, ns...);
}
template <typename T> void getv1(T &i) { cin >> i; }
template <typename T, typename... Ns>
void getv1(vector<T> &v, int n, Ns... ns) {
  v.resize(n + 1);
  for (int i = 1; i <= n; i++)
    getv(v[i], ns...);
}
#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
inline void getch(char &x) {
  while (x = getchar_unlocked(), x < 33) {
    ;
  }
}
inline void getstr(string &str) {
  str.clear();
  char cur;
  while (cur = getchar_unlocked(), cur < 33) {
    ;
  }
  while (cur > 32) {
    str += cur;
    cur = getchar_unlocked();
  }
}
template <typename T> inline bool sc(T &num) {
  bool neg = 0;
  int c;
  num = 0;
  while (c = getchar_unlocked(), c < 33) {
    if (c == EOF)
      return false;
  }
  if (c == '-') {
    neg = 1;
    c = getchar_unlocked();
  }
  for (; c > 47; c = getchar_unlocked())
    num = num * 10 + c - 48;
  if (neg)
    num *= -1;
  return true;
}
typedef unsigned long long ull;
typedef long long ll;
typedef float ld;
typedef ll _I;
typedef pair<_I, _I> pi;
typedef pair<ld, ld> pd;
typedef map<_I, _I> mii;
typedef __V<_I> vi;
typedef __V<char> vc;
typedef __V<ld> vd;
typedef __V<vd> vvd;
typedef __V<pi> vpi;
typedef __V<__V<_I>> vvi;
typedef __V<__V<char>> vvc;
typedef __V<__V<pi>> vvpi;
using AntonTsypko = void;
using arsijo = AntonTsypko;
using god = arsijo;
uniform_real_distribution<double> double_dist(0, 1);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::steady_clock::now().time_since_epoch().count());
int main() {
  doin();
  string s;
  stack<char> a;
  cin >> s;
  int ans = 0;
  for (auto i : s) {
    if (!a.empty() && a.top() != i)
      a.pop(), ans += 2;
    else
      a.push(i);
  }
  cout << ans;

  return 0;
}