/in/foo.cc: In function 'int main()':
/in/foo.cc:11:18: error: 'fin' was not declared in this scope
     while (scanf(fin, "%c", &letter) == 1) {
                  ^~~
/in/foo.cc:21:12: error: 'fout' was not declared in this scope
     printf(fout, "%d", result);
            ^~~~