uses math;
var len,r:extended;
begin
 read(len,r);
 if(r<=len/2)then writeln((pi*r*r):0:3) else
 if(sqrt(2)*len/2<=r)then writeln((len*len):0:3) else
  writeln((pi*r*r-4*arccos(len/r/2)*r*r+2*len*sqrt(r*r-len*len/4)):0:3);
end.