foo.cc: In function 'void init()':
foo.cc:11:15: warning: iteration 100000008 invokes undefined behavior [-Waggressive-loop-optimizations]
         if(a[i]==0) prime[++cnt]=i;
            ~~~^
foo.cc:9:18: note: within this loop
     for(int i=2;i<=maxn;i++)
                 ~^~~~~~

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