using System;

class B
{
	static int[] Read() => Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
	static (int, int, int, int) Read4() { var a = Read(); return (a[0], a[1], a[2], a[3]); }
	static void Main()
	{
		var (x, y, z, w) = Read4();
		Console.WriteLine((x - y * z) / 2.0);
	}
}