foo.cc: In function 'int main()':
foo.cc:5:12: warning: unused variable 'b' [-Wunused-variable]
    5 |  int i, a, b, gcd, s = 0;
      |            ^
foo.cc:6:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    6 |  scanf ("%d", &a);
      |  ~~~~~~^~~~~~~~~~

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