Problem p03569 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, n) for (int64_t i = 0; i < (int64_t)(n); i++)
using namespace std;

//////全探索したいときに開けよ////////
/*
vector<vector<int>> fspattern;
vector<int> fspattern_draft;
int fspcnt = 0;

void setfspattern_core(int floor,int number,int degree){
  if(floor==degree){
    fspattern.at(fspcnt) = fspattern_draft;
    fspcnt++;
  }else{
    rep(i,number){
      fspattern_draft.at(floor) = i;
      setfspattern_core(floor+1,number,degree);
    }
  }
}

void setfspattern(int number,int degree){
  int64_t array_num = 1;
  rep(i,degree){
    array_num *= number;
  }
  fspattern = vector<vector<int>>(array_num,vector<int>(degree));
  fspattern_draft = vector<int>(degree);
  setfspattern_core(0,number,degree);
}
*/

//////nCrのmodを求めたいときに開けよ/////////
// int64_t univMod = 1000000007;
// int64_t factnum = 100000
// vector<int64_t> modfact(factnum);
// vector<int64_t> modinvfact(factnum);

/*void setmodfact(){
  for(int i=0;i<factnum;i++){
    if(i == 0){
      modfact.at(i) = 1;
    }else{
      modfact.at(i) = (modfact.at(i-1)*i)%univMod;
    }
  }
}

int64_t calcmodpower(int64_t a,int64_t n){
  int64_t res = 1;
  while(n != 0){
    if(n & 1){
      res = (res * a) % univMod;
    }
    a = (a*a) % univMod;
    n = n >> 1;
  }
  return res;
}

int64_t calcinverse(int64_t n){
        return calcmodpower(n,univMod-2);
}

void setmodinvfact(){
  for(int i=0;i<factnum;i++){
    if(i==0){
      modinvfact.at(i) = 1;
    }else{
      modinvfact.at(i) = calcmodpower(modfact.at(i),univMod-2);
    }
  }
}*/

/*
int64_t calcGCD(int64_t a,int64_t b){
        int64_t p = a;
        int64_t q = b;
        int64_t c = a%b;
        while(c != 0){
                p = q;
                q = c;
                c = p%q;
        }
        return q;
}
*/

int main() {
  string s;
  cin >> s;
  string wx;
  rep(i, s.size()) {
    if (s.at(i) != 'x') {
      wx.push_back(s.at(i));
    }
  }
  // cout << wx;
  string wxr = wx;
  reverse(wx.begin(), wx.end());
  int result = 0;
  if (wxr != wx) {
    cout << -1 << endl;
  } else {
    int l = 0;
    int r = s.size() - 1;
    while (l != r) {
      if (s.at(l) == s.at(r)) {
        l++;
        r--;
      } else {
        if (s.at(l) == 'x') {
          l++;
          result++;
        }
        if (s.at(r) == 'x') {
          r--;
          result++;
        }
      }
    }
    cout << result << endl;
  }
}


Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int64_t i = 0; i < (int64_t)(n); i++)
using namespace std;

//////全探索したいときに開けよ////////
/*
vector<vector<int>> fspattern;
vector<int> fspattern_draft;
int fspcnt = 0;

void setfspattern_core(int floor,int number,int degree){
  if(floor==degree){
    fspattern.at(fspcnt) = fspattern_draft;
    fspcnt++;
  }else{
    rep(i,number){
      fspattern_draft.at(floor) = i;
      setfspattern_core(floor+1,number,degree);
    }
  }
}

void setfspattern(int number,int degree){
  int64_t array_num = 1;
  rep(i,degree){
    array_num *= number;
  }
  fspattern = vector<vector<int>>(array_num,vector<int>(degree));
  fspattern_draft = vector<int>(degree);
  setfspattern_core(0,number,degree);
}
*/

//////nCrのmodを求めたいときに開けよ/////////
// int64_t univMod = 1000000007;
// int64_t factnum = 100000
// vector<int64_t> modfact(factnum);
// vector<int64_t> modinvfact(factnum);

/*void setmodfact(){
  for(int i=0;i<factnum;i++){
    if(i == 0){
      modfact.at(i) = 1;
    }else{
      modfact.at(i) = (modfact.at(i-1)*i)%univMod;
    }
  }
}

int64_t calcmodpower(int64_t a,int64_t n){
  int64_t res = 1;
  while(n != 0){
    if(n & 1){
      res = (res * a) % univMod;
    }
    a = (a*a) % univMod;
    n = n >> 1;
  }
  return res;
}

int64_t calcinverse(int64_t n){
        return calcmodpower(n,univMod-2);
}

void setmodinvfact(){
  for(int i=0;i<factnum;i++){
    if(i==0){
      modinvfact.at(i) = 1;
    }else{
      modinvfact.at(i) = calcmodpower(modfact.at(i),univMod-2);
    }
  }
}*/

/*
int64_t calcGCD(int64_t a,int64_t b){
        int64_t p = a;
        int64_t q = b;
        int64_t c = a%b;
        while(c != 0){
                p = q;
                q = c;
                c = p%q;
        }
        return q;
}
*/

int main() {
  string s;
  cin >> s;
  string wx;
  rep(i, s.size()) {
    if (s.at(i) != 'x') {
      wx.push_back(s.at(i));
    }
  }
  // cout << wx;
  string wxr = wx;
  reverse(wx.begin(), wx.end());
  int result = 0;
  if (wxr != wx) {
    cout << -1 << endl;
  } else {
    int l = 0;
    int r = s.size() - 1;
    while (l < r) {
      if (s.at(l) == s.at(r)) {
        l++;
        r--;
      } else {
        if (s.at(l) == 'x') {
          l++;
          result++;
        }
        if (s.at(r) == 'x') {
          r--;
          result++;
        }
      }
    }
    cout << result << endl;
  }
}