foo.cc: In function 'int main()':
foo.cc:5:9: error: 'cin' was not declared in this scope
    5 |         cin>>x;
      |         ^~~
foo.cc:15:17: error: 'cout' was not declared in this scope
   15 |                 cout<<sum<<endl;
      |                 ^~~~
foo.cc:15:28: error: 'endl' was not declared in this scope
   15 |                 cout<<sum<<endl;
      |                            ^~~~