foo.cc:5:5: error: cannot declare '::main' to be a global variable
 int main
     ^~~~
foo.cc:7:5: error: expected primary-expression before 'int'
     int a,s;
     ^~~
foo.cc:7:5: error: expected '}' before 'int'
foo.cc:8:5: error: 'cin' does not name a type; did you mean 'main'?
     cin>>a;
     ^~~
     main
foo.cc:9:5: error: 's' does not name a type
     s=a*a;
     ^
foo.cc:10:5: error: 'cout' does not name a type
     cout << s;
     ^~~~
foo.cc:12:5: error: expected unqualified-id before 'return'
     return 0;
     ^~~~~~
foo.cc:13:1: error: expected declaration before '}' token
 }
 ^

[Hydro](https://hydro.ac)提供评测服务