Language: C++

int main() {

	double A, B, C, K = 0;
	double max = 10e18;
	
	cin >> A >> B >> C >> K;


	std::vector<double> ABC;
	std::vector<double> tempABC(3);
	ABC.push_back(A);
	ABC.push_back(B);
	ABC.push_back(C);

	for (int i = 0; i < K; i++){
		for (int j = 0; j < 3; j++){
			tempABC[j] = ABC[(j + 1) % 3] + ABC[(j + 2) % 3];
		}
		ABC[0] = tempABC[0];
		ABC[1] = tempABC[1];
		ABC[2] = tempABC[2];
		if (abs(ABC[0] - ABC[1]) > max){
			std::cout << "Unfair" << std::endl;
			return 0;
		}
	}
	std::cout << ABC[0] - ABC[1] << std::endl;
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:6:9: error: 'cin' was not declared in this scope
    6 |         cin >> A >> B >> C >> K;
      |         ^~~
a.cc:9:14: error: 'vector' is not a member of 'std'
    9 |         std::vector<double> ABC;
      |              ^~~~~~
a.cc:1:1: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
  +++ |+#include <vector>
    1 | int main() {
a.cc:9:21: error: expected primary-expression before 'double'
    9 |         std::vector<double> ABC;
      |                     ^~~~~~
a.cc:10:14: error: 'vector' is not a member of 'std'
   10 |         std::vector<double> tempABC(3);
      |              ^~~~~~
a.cc:10:14: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
a.cc:10:21: error: expected primary-expression before 'double'
   10 |         std::vector<double> tempABC(3);
      |                     ^~~~~~
a.cc:11:9: error: 'ABC' was not declared in this scope
   11 |         ABC.push_back(A);
      |         ^~~
a.cc:17:25: error: 'tempABC' was not declared in this scope
   17 |                         tempABC[j] = ABC[(j + 1) % 3] + ABC[(j + 2) % 3];
      |                         ^~~~~~~
a.cc:19:26: error: 'tempABC' was not declared in this scope
   19 |                 ABC[0] = tempABC[0];
      |                          ^~~~~~~
a.cc:22:21: error: 'abs' was not declared in this scope
   22 |                 if (abs(ABC[0] - ABC[1]) > max){
      |                     ^~~
a.cc:23:30: error: 'cout' is not a member of 'std'
   23 |                         std::cout << "Unfair" << std::endl;
      |                              ^~~~
a.cc:1:1: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
  +++ |+#include <iostream>
    1 | int main() {
a.cc:23:55: error: 'endl' is not a member of 'std'
   23 |                         std::cout << "Unfair" << std::endl;
      |                                                       ^~~~
a.cc:1:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
  +++ |+#include <ostream>
    1 | int main() {
a.cc:27:14: error: 'cout' is not a member of 'std'
   27 |         std::cout << ABC[0] - ABC[1] << std::endl;
      |              ^~~~
a.cc:27:14: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:27:46: error: 'endl' is not a member of 'std'
   27 |         std::cout << ABC[0] - ABC[1] << std::endl;
      |                                              ^~~~
a.cc:27:46: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'