foo.cc: In function ‘int main()’:
foo.cc:6:9: error: ‘cin’ was not declared in this scope
    6 |         cin>>a;
      |         ^~~
foo.cc:7:24: error: ‘n’ was not declared in this scope
    7 |         for(int i=1;i<=n;i++){
      |                        ^
foo.cc:10:9: error: ‘cout’ was not declared in this scope
   10 |         cout<<sum;
      |         ^~~~