/in/foo.cc:51:16: error: 'istream' does not name a type
   51 |         friend istream& operator>>(istream& in, const Matrix3& other) {
      |                ^~~~~~~
/in/foo.cc:59:16: error: 'ostream' does not name a type
   59 |         friend ostream& operator<<(ostream& out,const Matrix3& other){
      |                ^~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:73:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
   73 |         cin>>k;
      |         ^~~
      |         std::cin
In file included from /in/foo.cc:1:
/usr/include/c++/12/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;           /// Linked to standard input
      |                  ^~~
/in/foo.cc:76:9: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   76 |         cout<<(m1+m2-m3)*k<<endl;
      |         ^~~~
      |         std::cout
/usr/include/c++/12/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;          /// Linked to standard output
      |                  ^~~~
/in/foo.cc:76:29: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   76 |         cout<<(m1+m2-m3)*k<<endl;
      |                             ^~~~
      |                             std::endl
In file included from /usr/include/c++/12/iostream:39:
/usr/include/c++/12/ostream:688:5: note: 'std::endl' declared here
  688 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~