Problem p02622 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define SORT(c) sort((c).begin(), (c).end());
#define pb push_back
#define MP make_pair
#define pii pair<int, int>
#define pcc pair<char, char>
#define pic pair<int, char>
#define pci pair<char, int>
#define VS vector<string>
#define VI vector<int>
#define pi 3.141592653589793
#define ll long long
#define ull unsigned long long
#define w(x)                                                                   \
  int x;                                                                       \
  cin >> x;                                                                    \
  while (x--)
#define fast ios_base::sync_with_stdio(0)
#define sz size()
#define cu cout <<
#define en endl
#define sp " "
#define RE(i, n) for (int i = 0, _n = (n); i < _n; ++i)
#define RI(i, n) for (int i = 1, _n = (n); i <= _n; ++i)
#include <cstdio>
#include <cstring>
typedef unsigned long long UL;
template <typename T> inline T S(T a) { return a * a; }
template <class T> inline string tostring(T a) {
  ostringstream os("");
  os << a;
  return os.str();
}
template <typename T> inline ll tolong(T a) {
  ll res;
  istringstream os(a);
  os >> res;
  return res;
}
template <typename T> inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
template <typename T> inline T bigmod(T a, T b, T m) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0)
    return S(bigmod(a, b / 2, m)) % m;
  else
    return (a % m * bigmod(a, b - 1, m)) % m;
}
const int inf = (int)1e9 + 5;
const ll linf = (ll)1e16 + 5;
const ll modd = (ll)1e9 + 7;
const int mod = 10000007;
/*#define N 20000009
bool prime[N];
vector<ll> prm;
void seive ()
{

    memset(prime, true, sizeof(prime));

    for( ll i=2; i*i<=N; i++)
    {
        if (prime[i] == true)
        {
            prm.pb(i);
            for (ll j=i*i; j<=N; j += i)
                pe[j] = false;
        }
    }
}*/
/*bool isprime(ll n)
{
    if(n<2) return 0;
    if(n==2) return 1;
    else
    {
        for(int i=2; i*i<=n; i++)
            if(n%i==0) return 0;
    }
    return 1;
}*/
void pr(int f) {
  if (f == 1)
    cout << "YES\n";
  else
    cout << "NO\n";
}

int main() {
  fast;
  string s, ss;
  cin >> s >> s;
  int c = 0;
  for (int i = 0; i < s.sz; i++) {
    if (s[i] != ss[i])
      c++;
  }
  cout << c << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define SORT(c) sort((c).begin(), (c).end());
#define pb push_back
#define MP make_pair
#define pii pair<int, int>
#define pcc pair<char, char>
#define pic pair<int, char>
#define pci pair<char, int>
#define VS vector<string>
#define VI vector<int>
#define pi 3.141592653589793
#define ll long long
#define ull unsigned long long
#define w(x)                                                                   \
  int x;                                                                       \
  cin >> x;                                                                    \
  while (x--)
#define fast ios_base::sync_with_stdio(0)
#define sz size()
#define cu cout <<
#define en endl
#define sp " "
#define RE(i, n) for (int i = 0, _n = (n); i < _n; ++i)
#define RI(i, n) for (int i = 1, _n = (n); i <= _n; ++i)
#include <cstdio>
#include <cstring>
typedef unsigned long long UL;
template <typename T> inline T S(T a) { return a * a; }
template <class T> inline string tostring(T a) {
  ostringstream os("");
  os << a;
  return os.str();
}
template <typename T> inline ll tolong(T a) {
  ll res;
  istringstream os(a);
  os >> res;
  return res;
}
template <typename T> inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
template <typename T> inline T bigmod(T a, T b, T m) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0)
    return S(bigmod(a, b / 2, m)) % m;
  else
    return (a % m * bigmod(a, b - 1, m)) % m;
}
const int inf = (int)1e9 + 5;
const ll linf = (ll)1e16 + 5;
const ll modd = (ll)1e9 + 7;
const int mod = 10000007;
/*#define N 20000009
bool prime[N];
vector<ll> prm;
void seive ()
{

    memset(prime, true, sizeof(prime));

    for( ll i=2; i*i<=N; i++)
    {
        if (prime[i] == true)
        {
            prm.pb(i);
            for (ll j=i*i; j<=N; j += i)
                pe[j] = false;
        }
    }
}*/
/*bool isprime(ll n)
{
    if(n<2) return 0;
    if(n==2) return 1;
    else
    {
        for(int i=2; i*i<=n; i++)
            if(n%i==0) return 0;
    }
    return 1;
}*/
void pr(int f) {
  if (f == 1)
    cout << "YES\n";
  else
    cout << "NO\n";
}

int main() {
  fast;
  string s, ss;
  cin >> s >> ss;
  int c = 0;
  for (int i = 0; i < s.sz; i++) {
    if (s[i] != ss[i])
      c++;
  }
  cout << c << endl;
}