foo.cc: In function 'int main()':
foo.cc:28:9: warning: unused variable 'xx1' [-Wunused-variable]
  BigInt xx1;
         ^~~
foo.cc:29:8: warning: unused variable 'j' [-Wunused-variable]
  int v,j,k;
        ^
foo.cc:18:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d%d%d",&m1,&n1,&m2,&n2);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

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