/in/foo.cc: In function 'int main()':
/in/foo.cc:6:13: error: expected unqualified-id before 'int'
  int a[n+9],int b[n+9];
             ^~~
/in/foo.cc:8:14: error: 'b' was not declared in this scope
   cin>>a[i]>>b[i];
              ^
/in/foo.cc:11:9: error: 'b' was not declared in this scope
   a[i]*=b[i];
         ^