# Orchestra

## 题意翻译

- 在一个 $r \times c$ 的矩阵中有 $n$ 个点，问有多少个连续子矩阵至少包含 $k$ 个点。
- $r,c,n,k \le 10$。

## 题目描述

Paul is at the orchestra. The string section is arranged in an $ r×c $ rectangular grid and is filled with violinists with the exception of $ n $ violists. Paul really likes violas, so he would like to take a picture including at least $ k $ of them. Paul can take a picture of any axis-parallel rectangle in the orchestra. Count the number of possible pictures that Paul can take.
Two pictures are considered to be different if the coordinates of corresponding rectangles are different.

## 输入输出格式

### 输入格式

The first line of input contains four space-separated integers $ r $ , $ c $ , $ n $ , $ k $ ( $ 1<=r,c,n<=10 $ , $ 1<=k<=n $ ) — the number of rows and columns of the string section, the total number of violas, and the minimum number of violas Paul would like in his photograph, respectively.
The next $ n $ lines each contain two integers $ x_{i} $ and $ y_{i} $ ( $ 1<=x_{i}<=r $ , $ 1<=y_{i}<=c $ ): the position of the $ i $ -th viola. It is guaranteed that no location appears more than once in the input.

### 输出格式

Print a single integer — the number of photographs Paul can take which include at least $ k $ violas.

## 输入输出样例

### 输入样例 #1

```
2 2 1 1
1 2
```

### 输出样例 #1

```
4
```

### 输入样例 #2

```
3 2 3 3
1 1
3 1
2 2
```

### 输出样例 #2

```
1
```

### 输入样例 #3

```
3 2 3 2
1 1
3 1
2 2
```

### 输出样例 #3

```
4
```

## 说明

We will use '\*' to denote violinists and '\#' to denote violists.
In the first sample, the orchestra looks as follows
`<br></br>*#<br></br>**<br></br>` Paul can take a photograph of just the viola, the $ 1×2 $ column containing the viola, the $ 2×1 $ row containing the viola, or the entire string section, for $ 4 $ pictures total.In the second sample, the orchestra looks as follows
`<br></br>#*<br></br>*#<br></br>#*<br></br>` Paul must take a photograph of the entire section.In the third sample, the orchestra looks the same as in the second sample.