foo.cc: In function 'int main(int, char**)':
foo.cc:11:12: warning: unused variable 'k' [-Wunused-variable]
  int i, j, k;
            ^
foo.cc: In function 'int Mul(BigInt*, int)':
foo.cc:68:14: warning: unused variable 'k' [-Wunused-variable]
  int jw = 0, k = 0;
              ^
foo.cc: In function 'int main(int, char**)':
foo.cc:16:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &n1, &n2);
  ~~~~~^~~~~~~~~~~~~~~~~~~
foo.cc:17:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &m1, &m2);
  ~~~~~^~~~~~~~~~~~~~~~~~~
foo.cc:19:10: warning: 'nb' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int na, nb;
          ^~
foo.cc:19:6: warning: 'na' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int na, nb;
      ^~

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