foo.cc: In function 'int main()':
foo.cc:28:17: warning: unused variable 'j' [-Wunused-variable]
  register int i,j,p1,p2;
                 ^
foo.cc:29:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ~~~~~^~~~~~~~~

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