Language: C++

#include <bits/stdc++.h>

using namespace std;
typedef complex<double> P;
typedef vector<P> Polygon;

#define rep(i,n) for(int i=a;i<(int)0;i++)

#define prev(G, i) ( G[(i-1 + G.size()) % G.size()] )
#define curr(G, i) ( G[i] )
#define next(G, i) ( G[(i+1) % G.size()] )

double cross(P const& a, P const& b) {
  return imag(conj(a)*b);
}

double area(Polygon const& G) {
  double A = 0;
  for (int i=0; i<(int)G.size(); i++) 
    A += cross(curr(G, i), next(G, i));
  return A/2.;
}

int main() {
  
  int N; cin >> N;
  Polygon G(N);
  rep(i, N) cin >> G[i].real() >> G[i].imag();
  printf("%.10f\n", area(G));
  
  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:28: error: 'a' was not declared in this scope
    7 | #define rep(i,n) for(int i=a;i<(int)0;i++)
      |                            ^
a.cc:28:3: note: in expansion of macro 'rep'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |   ^~~
a.cc:28:17: error: no match for 'operator>>' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and 'double')
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |             ~~~ ^~ ~~~~~~~~~~~
      |             |               |
      |             |               double
      |             std::istream {aka std::basic_istream<char>}
In file included 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,
                 from a.cc:1:
/usr/include/c++/14/istream:170:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(bool&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  170 |       operator>>(bool& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:170:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'bool&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:174:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(short int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  174 |       operator>>(short& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:174:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'short int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:177:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(short unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  177 |       operator>>(unsigned short& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:177:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'short unsigned int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:181:7: note: candidate: 'std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(int&) [with _CharT = char; _Traits = std::char_traits<char>]' (near match)
  181 |       operator>>(int& __n);
      |       ^~~~~~~~
/usr/include/c++/14/istream:181:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:184:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  184 |       operator>>(unsigned int& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:184:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'unsigned int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:188:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  188 |       operator>>(long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:188:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'long int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:192:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  192 |       operator>>(unsigned long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:192:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'long unsigned int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:199:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  199 |       operator>>(long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:199:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'long long int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:203:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long long unsigned int&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  203 |       operator>>(unsigned long long& __n)
      |       ^~~~~~~~
/usr/include/c++/14/istream:203:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'long long unsigned int&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:219:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(float&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  219 |       operator>>(float& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:219:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'float&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:223:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  223 |       operator>>(double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:223:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'double&' to an rvalue of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:227:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(long double&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]' (near match)
  227 |       operator>>(long double& __f)
      |       ^~~~~~~~
/usr/include/c++/14/istream:227:7: note:   conversion of argument 1 would be ill-formed:
a.cc:28:29: error: cannot bind non-const lvalue reference of type 'long double&' to a value of type 'double'
   28 |   rep(i, N) cin >> G[i].real() >> G[i].imag();
      |                    ~~~~~~~~~^~
/usr/include/c++/14/istream:328:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(void*&) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]'
  328 |       operator>>(void*& __p)
      |       ^~~~~~~~
/usr/include/c++/14/istream:328:25: note:   no known conversion for argument 1 from 'double' to 'void*&'
  328 |       operator>>(void*& __p)
      |                  ~~~~~~~^~~
/usr/include/c++/14/istream:122:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__istream_type& (*)(__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>]'
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:122:36: note:   no known conversion for argument 1 from 'double' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'}
  122 |       operator>>(__istream_type& (*__pf)(__istream_type&))
      |                  ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/istream:126:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(__ios_type& (*)(__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __istream_type = std::basic_istream<char>; __ios_type = std::basic_ios<char>]'
  126 |       operator>>(__ios_type& (*__pf)(__ios_type&))
      |       ^~~~~~~~
/usr/include/c++/14/istream:126:32: note:   no known conversion for argument 1 from 'double' to 'std::basic_istream<char>::__ios_type& (*)(std::bas