/in/foo.cc: In function 'int main()':
/in/foo.cc:29:31: warning: iteration 100006 invokes undefined behavior [-Waggressive-loop-optimizations]
     for(int i=1;i<=M;i++)fa[i]=i;
                          ~~~~~^~
/in/foo.cc:29:18: note: within this loop
     for(int i=1;i<=M;i++)fa[i]=i;
                  ^