Language: C++

// I make this just for fun because i'm done
// Aimi Haraguni >> Konomi Suzuki >> Yui >>  Ikimono Gakari >> Garnidelia >> Kalafina >> Eir Aoi.   .. dude? 
// problems that involves any kind of persistent data structures are the best of the best, are not them?
 
// #pragma GCC optimize ("Ofast,unroll-loops")
// #pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
 
#include <bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
 
#define pb emplace_back
#define ff  first
#define ss second
#define tm1 first
#define tm2 second.first
#define tm3 second.second
#define sz(x) ll(x.size())
#define all(v) (v).begin(), (v).end()
#define FER(i,a,b) for(ll i=ll(a); i< ll(b); ++i)
#define IFR(i,a,b) for(ll i=ll(a); i>=ll(b); --i )
#define fastio ios_base::sync_with_stdio(0); cin.tie(0)
 
#define N 31000000
// #define M 19
 
// #define mod  11092019
#define mod1 1000000007
#define mod2 1000000009
#define bas 987625403
#define sqr(x) (x)*(x)  
#define INF 2000000000
 
using namespace std;
using namespace __gnu_pbds;
 
typedef  ll;
typedef pair<ll, ll> ii;
typedef pair<ll, ii > tri;
typedef vector<ll> vi;
typedef vector<ii> vii;
typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
 
#define trace(...) fff(#__VA_ARGS__, __VA_ARGS__)
template<typename t> void fff(const char* x, t&& val1) { cout<<x<< " : "<<val1<<"\n";}
template<typename t1, typename... t2> void fff(const char* x, t1&& val1, t2&&... val2){
    const char* xd=strchr(x+1, ',');
    cout.write(x, xd-x)<<" : "<<val1<<" | ";
    fff(xd+1, val2...);
}

inline ll mul(ll a, ll b, ll mod) { return (long long) a*b%mod;}
inline ll add(ll a, ll b, ll mod) { return a+b < mod? a+b: a+b-mod;}
inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
inline ll bp(ll a, ll p, ll mod){
	ll ret;
	for(ret = 1; p; p>>=1, Mul(a, a, mod)) (p & 1) && (Mul(ret, a, mod), 1);
	return ret;
}

static inline void read(ll &result) {
    bool minus = false;
    char ch;
    ch = getchar();
    while (true) {
        if (ch == '-') break;
        if (ch >= '0' and ch <= '9') break;
        ch = getchar();
    }
    (ch == '-')? minus = true: result = ch-'0';
    while (true) {
        ch = getchar();
        if (ch < '0' or ch > '9') break;
        result = (result<<3)+ (result<<1) + (ch - '0');
    }
    if(minus) result=-result;
}

int main(){
	fastio;
	string a, b; cin>>a>>b;
	ll ans = 0;
	FER(i, 0, sz(a)){
		if(a[i] != b[i]) ans++;
	}
	cout<<ans<<"\n";
	return 0;
}

Compiler output:
a.cc:36:10: error: 'll' does not name a type; did you mean 'all'?
   36 | typedef  ll;
      |          ^~
      |          all
a.cc:37:14: error: 'll' was not declared in this scope; did you mean 'all'?
   37 | typedef pair<ll, ll> ii;
      |              ^~
      |              all
a.cc:37:18: error: 'll' was not declared in this scope; did you mean 'all'?
   37 | typedef pair<ll, ll> ii;
      |                  ^~
      |                  all
a.cc:37:20: error: template argument 1 is invalid
   37 | typedef pair<ll, ll> ii;
      |                    ^
a.cc:37:20: error: template argument 2 is invalid
a.cc:38:14: error: 'll' was not declared in this scope; did you mean 'all'?
   38 | typedef pair<ll, ii > tri;
      |              ^~
      |              all
a.cc:38:21: error: template argument 1 is invalid
   38 | typedef pair<ll, ii > tri;
      |                     ^
a.cc:39:16: error: 'll' was not declared in this scope; did you mean 'all'?
   39 | typedef vector<ll> vi;
      |                ^~
      |                all
a.cc:39:18: error: template argument 1 is invalid
   39 | typedef vector<ll> vi;
      |                  ^
a.cc:39:18: error: template argument 2 is invalid
a.cc:41:19: error: 'll' was not declared in this scope; did you mean 'all'?
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                   ^~
      |                   all
a.cc:41:23: error: 'll' was not declared in this scope; did you mean 'all'?
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                       ^~
      |                       all
a.cc:41:25: error: template argument 1 is invalid
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                         ^
a.cc:41:25: error: template argument 2 is invalid
a.cc:41:49: error: 'll' was not declared in this scope; did you mean 'all'?
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                                                 ^~
      |                                                 all
a.cc:41:53: error: 'll' was not declared in this scope; did you mean 'all'?
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                                                     ^~
      |                                                     all
a.cc:41:53: error: template argument 1 is invalid
a.cc:41:53: error: template argument 2 is invalid
a.cc:41:55: error: template argument 1 is invalid
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                                                       ^~
a.cc:41:105: error: template argument 1 is invalid
   41 | typedef tree<pair<ll, ll>, null_type, less<pair<ll, ll>>, rb_tree_tag, tree_order_statistics_node_update> S_t;
      |                                                                                                         ^
a.cc:41:105: error: template argument 3 is invalid
a.cc:52:8: error: 'll' does not name a type; did you mean 'all'?
   52 | inline ll mul(ll a, ll b, ll mod) { return (long long) a*b%mod;}
      |        ^~
      |        all
a.cc:53:8: error: 'll' does not name a type; did you mean 'all'?
   53 | inline ll add(ll a, ll b, ll mod) { return a+b < mod? a+b: a+b-mod;}
      |        ^~
      |        all
a.cc:54:13: error: variable or field 'Mul' declared void
   54 | inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
      |             ^~~
a.cc:54:17: error: 'll' was not declared in this scope; did you mean 'all'?
   54 | inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
      |                 ^~
      |                 all
a.cc:54:21: error: 'a' was not declared in this scope
   54 | inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
      |                     ^
a.cc:54:24: error: 'll' was not declared in this scope; did you mean 'all'?
   54 | inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
      |                        ^~
      |                        all
a.cc:54:30: error: 'll' was not declared in this scope; did you mean 'all'?
   54 | inline void Mul(ll &a, ll b, ll mod) { a = (long long) a*b%mod;}
      |                              ^~
      |                              all
a.cc:55:8: error: 'll' does not name a type; did you mean 'all'?
   55 | inline ll bp(ll a, ll p, ll mod){
      |        ^~
      |        all
a.cc:61:20: error: variable or field 'read' declared void
   61 | static inline void read(ll &result) {
      |                    ^~~~
a.cc:61:25: error: 'll' was not declared in this scope; did you mean 'all'?
   61 | static inline void read(ll &result) {
      |                         ^~
      |                         all
a.cc:61:29: error: 'result' was not declared in this scope
   61 | static inline void read(ll &result) {
      |                             ^~~~~~
a.cc: In function 'int main()':
a.cc:82:9: error: 'll' was not declared in this scope; did you mean 'all'?
   82 |         ll ans = 0;
      |         ^~
      |         all
a.cc:83:13: error: expected ';' before 'i'
   83 |         FER(i, 0, sz(a)){
      |             ^
a.cc:19:27: note: in definition of macro 'FER'
   19 | #define FER(i,a,b) for(ll i=ll(a); i< ll(b); ++i)
      |                           ^
a.cc:83:13: error: 'i' was not declared in this scope
   83 |         FER(i, 0, sz(a)){
      |             ^
a.cc:19:36: note: in definition of macro 'FER'
   19 | #define FER(i,a,b) for(ll i=ll(a); i< ll(b); ++i)
      |                                    ^
a.cc:84:34: error: 'ans' was not declared in this scope; did you mean 'abs'?
   84 |                 if(a[i] != b[i]) ans++;
      |                                  ^~~
      |                                  abs
a.cc:86:15: error: 'ans' was not declared in this scope; did you mean 'abs'?
   86 |         cout<<ans<<"\n";
      |               ^~~
      |               abs