#描述#
<p>ChinaJoy又开始了！这次各大公司又将挂起各种宣传横幅。横幅会挂在两个柱子间的长度介于L1..L2的细线上。<p>
<p>ChinaJoy会场是一个W×H 的矩阵 并且主办方在每个坐标点上都竖立了柱子，在这 (W + 1) * (H + 1)个柱子上，主办方要选其中两个连上细线以挂上主办方自己的横幅。</p>
<p>CJ主办方不希望在他们的横幅之间有任何杂物，就是说在这条细线下面没有别的柱子，也不能穿过其它的柱子。</p>
<p>现在CJ主办方希望你能编程帮助他们算出有多少种挂横幅的方案。</p>
<p>考虑如下例子：</p>
<p>会场地图大小是W = 2 H = 1：</p>
<p>
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp*&nbsp*&nbsp*<br />
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp*&nbsp*&nbsp*<br />
</p>
<p>而横幅长度为2和3之间。这个会场共有 (2+1) * (1+1) = 6个点以及有(6 选 2) = (6*5)/(2*1) = 15 对不同的选择方法：</p>
<p>
&nbsp&nbsp&nbsp(0,0)-(0,1)&nbsp&nbsp&nbsp(0,0)-(2,1)&nbsp&nbsp&nbsp(0,1)-(2,1)&nbsp&nbsp&nbsp(1,1)-(2,0)<br />
&nbsp&nbsp&nbsp(0,0)-(1,0)&nbsp&nbsp&nbsp(0,1)-(1,0)&nbsp&nbsp&nbsp(1,0)-(1,1)&nbsp&nbsp&nbsp(1,1)-(2,1)<br />
&nbsp&nbsp&nbsp(0,0)-(1,1)&nbsp&nbsp&nbsp(0,1)-(1,1)&nbsp&nbsp&nbsp(1,0)-(2,0)&nbsp&nbsp&nbsp(2,0)-(2,1)<br />
&nbsp&nbsp&nbsp(0,0)-(2,0)&nbsp&nbsp&nbsp(0,1)-(2,0)&nbsp&nbsp&nbsp(1,0)-(2,1)<br />
</p>
<p>
在这之中，只有四种是长度在2到3之间的：</p>
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp长度&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp长度&nbsp<br />
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp(0,0)-(2,0)&nbsp2.00&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp(0,1)-(2,0)&nbsp2.24&nbsp<br />
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp(0,0)-(2,1)&nbsp2.24&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp(0,1)-(2,1)&nbsp2.00&nbsp<br />
<p>
但在这四种之中，(0,0)-(2,0)和(0,1)-(2,1)都穿过了其他的柱子，不符合主办方的要求，所以这个例子中15对选择方法中只有2种是可以成功挂上横幅的，所以答案是2。
</p>

#格式#
##输入格式##
输入有多组数据，每组数据一行，包含4个整数W， H，(1 &lt= W &lt= 1,000; 1 &lt= H &lt= 1,000)， L1, 和 L2,(1 &lt= L1 &lt= L2; L1 &lt= L2 &lt= 1,500)

##输出格式##
对于每组数据，输出一行，包含一个整数，表示可以成功帮主办方挂上横幅的方案有多少种。

#样例1#
##样例输入1##

2 1 2 3

##样例输出1##

2 

#限制#
1000ms
32768KB

#提示#

#来源#
LCS