foo.cc: In function 'int main()':
foo.cc:8:13: error: 'sum' was not declared in this scope
    8 |             sum += i;
      |             ^~~
foo.cc:11:13: error: 'sum' was not declared in this scope
   11 |             sum -= i;
      |             ^~~
foo.cc:15:12: error: 'sum' was not declared in this scope
   15 |     cout <<sum;
      |            ^~~

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