foo.cc: In function 'int main()':
foo.cc:7:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&k);
  ~~~~~^~~~~~~~~
foo.cc:9:11: warning: iteration 2000005 invokes undefined behavior [-Waggressive-loop-optimizations]
   if (!b[i])
        ~~~^
foo.cc:8:16: note: within this loop
  for (int i=2;i<=maxn;++i){
               ~^~~~~~

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