/in/foo.cc: In function 'int main()':
/in/foo.cc:6:2: error: 'cin' was not declared in this scope
  cin>>n;
  ^~~
/in/foo.cc:7:12: error: expected ')' before '!' token
   while(n--!){
            ^
/in/foo.cc:7:13: error: expected primary-expression before ')' token
   while(n--!){
             ^
/in/foo.cc:5:8: warning: unused variable 'a' [-Wunused-variable]
  int n,a;
        ^