Wrong Answer
/in/foo.cc: In function 'void Std::get_prime()':
/in/foo.cc:13:23: warning: iteration 49999 invokes undefined behavior [-Waggressive-loop-optimizations]
if(!mark[i])
~~~~~~^
/in/foo.cc:11:21: note: within this loop
for(ll i=2;i<=50001;i++)
~^~~~~~~