Language: C++

#include <bits/stdc++.h>

#define ld long double
#define ll long long int
#define ull unsigned long long int
#define vi vector<int>
#define vl vector<ll>
#define vvi vector< vector<int> >
#define vvl vector< vector<ll> >
#define repd(i, a, b) for (int i=(a);i<(b);i++)
#define rep(i, n) repd(i,0,n)
#define ALL(v) v.begin(), v.end()
#define INF 1e9

using namespace std;

/**
 * calculate GCD(greatest common divisor)
 * @param a
 * @param b
 * @return
 */
unsigned euclidean_gcd(unsigned a, unsigned b) {
    if (a < b) return euclidean_gcd(b, a);
    unsigned r;
    while ((r = a % b)) {
        a = b;
        b = r;
    }
    return b;
}

/**
 * change minimum
 * @tparam T
 * @param a
 * @param b
 * @return bool
 */
template<class T>
inline bool changeMinimum(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

/**
 * change maximum
 * @tparam T
 * @param a
 * @param b
 * @return bool
 */
template<class T>
inline bool changeMaximum(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

/**
 * output answer
 * @tparam T
 * @param answer
 * @return
 */
template<typename T>
T output(T answer) {
    cout << answer << endl;
    return 0;
}

/** ----from here ---------------------------------------------------------- */

int main() {
    string O, E;
    cin << O << E;
    int M;
    bool f = false;
    if (O.length() == E.length()) {
        M = O.length();
    } else {
        M = O.length() - 1;
        f = true;
    }

    for (int i = 0; i < M; ++i) {
        cout << O[i];
        cout << E[i];
    }
    if (f) { cout << O[M]; }
    cout << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:81:9: error: no match for 'operator<<' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
   81 |     cin << O << E;
      |     ~~~ ^~ ~
      |     |      |
      |     |      std::string {aka std::__cxx11::basic_string<char>}
      |     std::istream {aka std::basic_istream<char>}
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1715:5: note: candidate: 'template<class _Ch_type, class _Ch_traits, class _Bi_iter> std::basic_ostream<_CharT, _Traits>& std::__cxx11::operator<<(std::basic_ostream<_CharT, _Traits>&, const sub_match<_Bi_iter>&)'
 1715 |     operator<<(basic_ostream<_Ch_type, _Ch_traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1715:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:41:
/usr/include/c++/14/cstddef:125:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator<<(byte, _IntegerType)'
  125 |     operator<<(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:125:5: note:   template argument deduction/substitution failed:
a.cc:81:5: note:   cannot convert 'std::cin' (type 'std::istream' {aka 'std::basic_istream<char>'}) to type 'std::byte'
   81 |     cin << O << E;
      |     ^~~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:763:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, basic_string_view<_CharT, _Traits>)'
  763 |     operator<<(basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:763:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/bits/basic_string.h:4077:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 4077 |     operator<<(basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:4077:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/bitset:1687:5: note: candidate: 'template<class _CharT, class _Traits, long unsigned int _Nb> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const bitset<_Nb>&)'
 1687 |     operator<<(std::basic_ostream<_CharT, _Traits>& __os,
      |     ^~~~~~~~
/usr/include/c++/14/bitset:1687:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
In file included from /usr/include/c++/14/bits/ios_base.h:46,
                 from /usr/include/c++/14/streambuf:43,
                 from /usr/include/c++/14/bits/streambuf_iterator.h:35,
                 from /usr/include/c++/14/iterator:66,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:54:
/usr/include/c++/14/system_error:339:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const error_code&)'
  339 |     operator<<(basic_ostream<_CharT, _Traits>& __os, const error_code& __e)
      |     ^~~~~~~~
/usr/include/c++/14/system_error:339:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
In file included from /usr/include/c++/14/memory:80,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:56:
/usr/include/c++/14/bits/shared_ptr.h:70:5: note: candidate: 'template<class _Ch, class _Tr, class _Tp, __gnu_cxx::_Lock_policy _Lp> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const __shared_ptr<_Tp, _Lp>&)'
   70 |     operator<<(std::basic_ostream<_Ch, _Tr>& __os,
      |     ^~~~~~~~
/usr/include/c++/14/bits/shared_ptr.h:70:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
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:563:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, _CharT)'
  563 |     operator<<(basic_ostream<_CharT, _Traits>& __out, _CharT __c)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:563:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:573:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, char)'
  573 |     operator<<(basic_ostream<_CharT, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:573:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:579:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, char)'
  579 |     operator<<(basic_ostream<char, _Traits>& __out, char __c)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:579:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:590:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, signed char)'
  590 |     operator<<(basic_ostream<char, _Traits>& __out, signed char __c)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:590:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:595:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, unsigned char)'
  595 |     operator<<(basic_ostream<char, _Traits>& __out, unsigned char __c)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:595:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:654:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const _CharT*)'
  654 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:654:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
In file included from /usr/include/c++/14/ostream:1022:
/usr/include/c++/14/bits/ostream.tcc:307:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const char*)'
  307 |     operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/bits/ostream.tcc:307:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<_CharT, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:671:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(basic_ostream<char, _Traits>&, const char*)'
  671 |     operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/ostream:671:5: note:   template argument deduction/substitution failed:
a.cc:81:12: note:   'std::istream' {aka 'std::basic_istream<char>'} is not derived from 'std::basic_ostream<char, _Traits>'
   81 |     cin << O << E;
      |            ^
/usr/include/c++/14/ostream:684:5: note: candidate: 'template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(basic_ostre