func main()
	var x: int :: cui@inputInt()
	var y: int :: cui@inputInt()
	var z: int :: cui@inputInt()
	var w: int :: cui@inputInt()
	
	var ok: float :: 1.0e+10
	var ng: float :: 0.0
	for(1, 1000)
		var mid: float :: (ok + ng) / 2.0
		var bp: float :: lib@dist(ok, 0.0, 0.0, y $ float)
		var bq: float :: lib@dist(x $ float / ok, 0.0, 0.0, z $ float)
		if(bp - bq > w $ float)
			do ok :: mid
		else
			do ng :: mid
		end if
	end for
	
	var bc: float :: x $ float / ok
	var ans: float :: x $ float - (ok * y $ float + bc * z $ float + (bc - y $ float) * (ok - z $ float)) / 2.0
	do cui@print("\{ans.toStrFmt(".10f")}\n")
end func