/in/foo.cc: In function 'int main()':
/in/foo.cc:44:31: warning: variable 'ans' set but not used [-Wunused-but-set-variable]
  double l=0,r=n/(1.0+P*n),mid,ans;
                               ^~~
/in/foo.cc:94:23: warning: 'mid' may be used uninitialized in this function [-Wmaybe-uninitialized]
  printf("%.3lf\n",mid);
                       ^