Bus
Description
A bus moves along the coordinate lineOxfrom the pointx= 0 to the pointx=a. After starting from the pointx= 0, it reaches the pointx=a, immediately turns back and then moves to the pointx= 0. After returning to the pointx= 0 it immediately goes back to the pointx=aand so on. Thus, the bus moves fromx= 0 tox=aand back. Moving from the pointx= 0 tox=aor from the pointx=atox= 0 is called abus journey. In total, the bus must makekjourneys.
The petrol tank of the bus can holdbliters of gasoline. To pass a single unit of distance the bus needs to spend exactly one liter of gasoline. The bus starts its first journey with a full petrol tank.
There is a gas station in pointx=f. This point is between pointsx= 0 andx=a. There are no other gas stations on the bus route. While passing by a gas station in either direction the bus can stop and completely refuel its tank. Thus, after stopping to refuel the tank will containbliters of gasoline.
What is the minimum number of times the bus needs to refuel at the pointx=fto makekjourneys? The first journey starts in the pointx= 0.
公共汽车沿着坐标系牛X点 = 0 x点 = A.从X点 = 0后，它到达X点 = ，立即转身，然后移动到X点 = 0。返回点x = 0 立即回到X点 =
一等后。因此，从X = 0 X = 一回公交移动。从X点 = 0 X = 或从X点 = 一X = 0称为总线的旅程。总的来说，公共汽车必须进行K次旅行。
汽车的油箱能装B升汽油。为了通过一个单位的距离，公共汽车需要花整整一公升汽油。公共汽车开始满油箱的第一次旅程。
有点X = F.这一点加油站的点x = 0和x =
没有其他加油站的巴士路线之间。在任何方向经过加油站时，公共汽车都能停下来加油。因此，停下来加油后油箱里会含有B升的汽油。
时代的汽车需要加油点X = F使K旅行的最小的数是什么？第一次旅行在点x = 0 开始。
Format
Input
The first line contains four integersa,b,f,k(0 <f<a≤ e6,1 ≤b≤ e9,1 ≤k≤ e4) — the endpoint of the first bus journey, the capacity of the fuel tank of the bus, the point where the gas station is located, and the required number of journeys.
第一行包含四个整数A，B，F，K（0 ＜f＜≤ e6, 1 ≤B≤ e9, 1 ≤K≤ e4）-第一巴士旅程的终点，汽车的油箱容量，点到加油站的位置，和所需数量的旅程。
Output
Print the minimum number of times the bus needs to refuel to makekjourneys. If it is impossible for the bus to makekjourneys, print-1.
打印巴士需要加油的最低次数，使K旅程。如果公共汽车使K的旅行是不可能的，打印1。
Samples
6 9 2 4
4