/in/foo.cc: In function 'int main()':
/in/foo.cc:36:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(er[i]==i)
      ~~~~~^~~
/in/foo.cc:33:9: warning: iteration 999999 invokes undefined behavior [-Waggressive-loop-optimizations]
    er[i]=i;
    ~~~~~^~
/in/foo.cc:32:15: note: within this loop
  for(int i=2;i<=1000001;++i)
              ~^~~~~~~~~
/in/foo.cc:36:10: warning: iteration 999999 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(er[i]==i)
      ~~~~^
/in/foo.cc:34:15: note: within this loop
  for(int i=2;i<=1000001;++i)
              ~^~~~~~~~~