Problem:
Score : 500 points

Problem Statement

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.

Constraints

2 \leq N \times M \leq 2 \times 10^5

2 \leq K \leq N \times M

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M K

Output

Print the sum of the costs of all possible arrangements of the pieces, modulo 10^9+7.

Sample Input 1

2 2 2

Sample Output 1

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 Input 2

4 5 4

Sample Output 2

87210

Sample Input 3

100 100 5000

Sample Output 3

817260251

Be sure to print the sum modulo 10^9+7.

Solution in Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

func out(x ...interface{}) {
	fmt.Println(x...)
}

var sc = bufio.NewScanner(os.Stdin)

func getInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func getString() string {
	sc.Scan()
	return sc.Text()
}

// min, max, asub, absなど基本関数
func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func asub(a, b int) int {
	if a > b {
		return a - b
	}
	return b - a
}

func abs(a int) int {
	if a >= 0 {
		return a
	}
	return -a
}

func lowerBound(a []int, x int) int {
	idx := sort.Search(len(a), func(i int) bool {
		return a[i] >= x
	})
	return idx
}

func upperBound(a []int, x int) int {
	idx := sort.Search(len(a), func(i int) bool {
		return a[i] > x
	})
	return idx
}

const mod = 1000000007

func mpow(a, b int) int {
	if b == 0 {
		return 1
	} else if b%2 == 0 {
		tmp := mpow(a, b/2)
		return tmp * tmp % mod
	}
	return mpow(a, b-1) * a % mod
}

var fracMemo = []int{1, 1}

func mfrac(n int) int {
	if len(fracMemo) > n {
		return fracMemo[n]
	}
	if len(fracMemo) == 0 {
		fracMemo = append(fracMemo, 1)
	}
	for len(fracMemo) <= n {
		size := len(fracMemo)
		fracMemo = append(fracMemo, fracMemo[size-1]*size%mod)
	}
	return fracMemo[n]
}

var ifracMemo = []int{1, 1}

func mifrac(n int) int {
	if len(ifracMemo) > n {
		return ifracMemo[n]
	}
	if len(ifracMemo) == 0 {
		fracMemo = append(ifracMemo, 1)
	}
	for len(ifracMemo) <= n {
		size := len(ifracMemo)
		ifracMemo = append(ifracMemo, ifracMemo[size-1]*mpow(size, mod-2)%mod)
	}
	return ifracMemo[n]
}

func nCr(n, r int) int {
	if n == r {
		return 1
	}
	if n < r || r < 0 {
		return 0
	}
	ret := 1
	ret *= mfrac(n)
	ret %= mod
	ret *= mifrac(r)
	ret %= mod
	ret *= mifrac(n - r)
	ret %= mod
	return (ret)
}

func main() {
	sc.Split(bufio.ScanWords)
	N, M, K := getInt(), getInt(), getInt()

	ans := 0
	for d := 1; d < N; d++ {
		cnt := d * (N - d)
		cnt %= mod
		cnt *= M
		cnt %= mod
		cnt *= M
		cnt %= mod
		ans += cnt
		ans %= mod
	}
	for d := 1; d < M; d++ {
		cnt := d * (M - d)
		cnt %= mod
		cnt *= N
		cnt %= mod
		cnt *= N
		cnt %= mod
		ans += cnt
		ans %= mod
	}
	ans *= nCr(N*M-2, K-2)
	ans %= mod
	out(ans)
}