/in/foo.cc: In function 'int main()':
/in/foo.cc:30:3: error: 'f' was not declared in this scope
   f[i] = i * f[i - 1] + (i & 1 ? -1 : 1);
   ^
/in/foo.cc:31:10: error: 'f' was not declared in this scope
  cout << f[n];
          ^