Language: C++

//C
#include <cassert>
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>

//C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>

//Others
template<class T> inline bool chmin(T& a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
template<class T> inline bool chmax(T& a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}
const int dx[8] = { -1, 0, 1, -1, 1, -1, 0, 1 };
const int dy[8] = { 1, 1, 1, 0, 0, -1, -1, -1 };
const int lr[4] = { 0, 1, 0, -1 };
const int ud[4] = { 1, 0, -1, 0 };

const double PI = 3.14159265358979323846


//LOVE Ayane Sakura



using namespace std;

long long GCD(long long a, long long b) {
	if (a < b) {
		swap(a, b);
	}
	long long r = -1;
	while (r != 0) {
		r = a % b;
		a = b;
		b = r;
	}
	return a;
}

int main() {
	double R;
	cin >> R;
	cout << setprecision(15) << 2 * R * 3.14159265358979 << endl;
}


Compiler output:
a.cc:106:1: error: expected ',' or ';' before 'using'
  106 | using namespace std;
      | ^~~~~
a.cc: In function 'long long int GCD(long long int, long long int)':
a.cc:110:17: error: 'swap' was not declared in this scope
  110 |                 swap(a, b);
      |                 ^~~~
a.cc:110:17: note: suggested alternatives:
In file included from /usr/include/c++/14/regex:68,
                 from a.cc:69:
/usr/include/c++/14/bits/regex.h:2230:5: note:   'std::__cxx11::swap'
 2230 |     swap(match_results<_Bi_iter, _Alloc>& __lhs,
      |     ^~~~
In file included from /usr/include/c++/14/bits/stl_pair.h:61,
                 from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/bits/specfun.h:43,
                 from /usr/include/c++/14/cmath:3906,
                 from a.cc:9:
/usr/include/c++/14/bits/move.h:226:5: note:   'std::swap'
  226 |     swap(_Tp& __a, _Tp& __b)
      |     ^~~~
/usr/include/c++/14/bits/move.h:226:5: note:   'std::swap'
In file included from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/ios:41,
                 from /usr/include/c++/14/istream:40,
                 from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from a.cc:18:
/usr/include/c++/14/bits/exception_ptr.h:229:5: note:   'std::__exception_ptr::swap'
  229 |     swap(exception_ptr& __lhs, exception_ptr& __rhs)
      |     ^~~~
a.cc: In function 'int main()':
a.cc:123:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
  123 |         cin >> R;
      |         ^~~
      |         std::cin
In file included from a.cc:37:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:124:9: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
  124 |         cout << setprecision(15) << 2 * R * 3.14159265358979 << 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:124:17: error: 'setprecision' was not declared in this scope; did you mean 'std::setprecision'?
  124 |         cout << setprecision(15) << 2 * R * 3.14159265358979 << endl;
      |                 ^~~~~~~~~~~~
      |                 std::setprecision
In file included from a.cc:34:
/usr/include/c++/14/iomanip:202:3: note: 'std::setprecision' declared here
  202 |   setprecision(int __n)
      |   ^~~~~~~~~~~~
a.cc:124:65: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
  124 |         cout << setprecision(15) << 2 * R * 3.14159265358979 << endl;
      |                                                                 ^~~~
      |                                                                 std::endl
In file included from /usr/include/c++/14/istream:41:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~