Language: C++

#include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
int main(){
  int n,i;
  cin>>n;
  string s,t;
  rep(i,n) cin>>s;
  rep(i,n) cin>>t;
  string str;
  rep(i,n){
    k=i/2;
    str[2k+1]=s[i];
    str[2k]=t[i];
  }
  cout<<str<<endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:5:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    5 |   cin>>n;
      |   ^~~
      |   std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:146,
                 from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:6:3: error: 'string' was not declared in this scope
    6 |   string s,t;
      |   ^~~~~~
a.cc:6:3: note: suggested alternatives:
In file included from /usr/include/c++/14/string:41,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/bits/stringfwd.h:77:33: note:   'std::string'
   77 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
/usr/include/c++/14/string:76:11: note:   'std::pmr::string'
   76 |     using string    = basic_string<char>;
      |           ^~~~~~
a.cc:7:17: error: 's' was not declared in this scope
    7 |   rep(i,n) cin>>s;
      |                 ^
a.cc:8:17: error: 't' was not declared in this scope
    8 |   rep(i,n) cin>>t;
      |                 ^
a.cc:9:9: error: expected ';' before 'str'
    9 |   string str;
      |         ^~~~
      |         ;
a.cc:11:5: error: 'k' was not declared in this scope
   11 |     k=i/2;
      |     ^
a.cc:12:5: error: 'str' was not declared in this scope; did you mean 'std'?
   12 |     str[2k+1]=s[i];
      |     ^~~
      |     std
a.cc:12:9: error: fixed-point types not supported in C++
   12 |     str[2k+1]=s[i];
      |         ^~
a.cc:12:15: error: 's' was not declared in this scope
   12 |     str[2k+1]=s[i];
      |               ^
a.cc:13:9: error: fixed-point types not supported in C++
   13 |     str[2k]=t[i];
      |         ^~
a.cc:13:13: error: 't' was not declared in this scope
   13 |     str[2k]=t[i];
      |             ^
a.cc:15:3: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   15 |   cout<<str<<endl;
      |   ^~~~
      |   std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:15:9: error: 'str' was not declared in this scope; did you mean 'std'?
   15 |   cout<<str<<endl;
      |         ^~~
      |         std
a.cc:15:14: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   15 |   cout<<str<<endl;
      |              ^~~~
      |              std::endl
In file included from /usr/include/c++/14/istream:41,
                 from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~