10693: ABC127 —— E - Cell Distance
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Description
We have a grid of squares with $N$ rows and $M$ columns. Let $(i, j)$ denote the square at the $i$-th row from the top and $j$-th column from the left. We will choose $K$ of the squares and put a piece on each of them.
If we place the $K$ pieces on squares $(x_1, y_1)$, $(x_2, y_2)$, ..., and $(x_K, y_K)$, the _cost_ of this arrangement is computed as:
$\sum_{i=1}^{K-1} \sum_{j=i+1}^K (|x_i - x_j| + |y_i - y_j|)$
Find the sum of the costs of all possible arrangements of the pieces. Since this value can be tremendous, print it modulo $10^9+7$.
We consider two arrangements of the pieces different if and only if there is a square that contains a piece in one of the arrangements but not in the other.
If we place the $K$ pieces on squares $(x_1, y_1)$, $(x_2, y_2)$, ..., and $(x_K, y_K)$, the _cost_ of this arrangement is computed as:
$\sum_{i=1}^{K-1} \sum_{j=i+1}^K (|x_i - x_j| + |y_i - y_j|)$
Find the sum of the costs of all possible arrangements of the pieces. Since this value can be tremendous, print it modulo $10^9+7$.
We consider two arrangements of the pieces different if and only if there is a square that contains a piece in one of the arrangements but not in the other.
Input
Input is given from Standard Input in the following format:
```
$N$ $M$ $K$
```
```
$N$ $M$ $K$
```
Output
Print the sum of the costs of all possible arrangements of the pieces, modulo $10^9+7$.
Constraints
- $2 \leq N \times M \leq 2 \times 10^5$
- $2 \leq K \leq N \times M$
- All values in input are integers.
- $2 \leq K \leq N \times M$
- All values in input are integers.
Sample 1 Input
2 2 2
Sample 1 Output
8
There are six possible arrangements of the pieces, as follows:
- $((1,1),(1,2))$, with the cost $|1-1|+|1-2| = 1$
- $((1,1),(2,1))$, with the cost $|1-2|+|1-1| = 1$
- $((1,1),(2,2))$, with the cost $|1-2|+|1-2| = 2$
- $((1,2),(2,1))$, with the cost $|1-2|+|2-1| = 2$
- $((1,2),(2,2))$, with the cost $|1-2|+|2-2| = 1$
- $((2,1),(2,2))$, with the cost $|2-2|+|1-2| = 1$
The sum of these costs is $8$.
Sample 2 Input
4 5 4
Sample 2 Output
87210
Sample 3 Input
100 100 5000
Sample 3 Output
817260251