foo.cc: In function 'void init_primes()':
foo.cc:10:17: warning: iteration 200001 invokes undefined behavior [-Waggressive-loop-optimizations]
   10 |         if(!st[i])primes[idx++]=i;
      |             ~~~~^
foo.cc:8:18: note: within this loop
    8 |     for(int i=2;i<=N;i++)
      |                 ~^~~

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