/in/foo.cc: In function 'int main()':
/in/foo.cc:11:8: warning: iteration 1000004 invokes undefined behavior [-Waggressive-loop-optimizations]
   uf[i]=i;
   ~~~~~^~
/in/foo.cc:10:18: note: within this loop
     for(int i=1;i<=M;i++)
                  ^