/in/foo.cc: In function 'int main()':
/in/foo.cc:13:9: warning: iteration 11111110 invokes undefined behavior [-Waggressive-loop-optimizations]
   Num[i]=0;
   ~~~~~~^~
/in/foo.cc:12:24: note: within this loop
  for(register int i=1;i<=20000000;++i)
                       ~^~~~~~~~~~
/in/foo.cc:20:13: warning: iteration 11111110 invokes undefined behavior [-Waggressive-loop-optimizations]
   sum+=Num[i];
        ~~~~~^
/in/foo.cc:19:22: note: within this loop
  for(int i=1, sum=0;i<=20000000;++i){
                     ~^~~~~~~~~~