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 Python (3.4.3) (judge verdict: Accepted)

class FactMod:

    def __init__(self, n, mod):
        self.mod = mod
        self.f = [1] * (n + 1)
        for i in range(1, n + 1):
            self.f[i] = self.f[i - 1] * i % mod

        self.inv = [pow(self.f[-1], mod - 2, mod)]
        for i in reversed(range(1, n + 1)):
            self.inv.append(self.inv[-1] * i % mod)
        self.inv.reverse()

    def fact(self, n):
        """
        :return: n!
        """
        return self.f[n]

    def comb(self, n, r):
        """
        :return: nCr
        """
        return self.f[n] * self.inv[n - r] * self.inv[r] % self.mod


N, M, K = map(int, input().split())
mod = 10 ** 9 + 7

NM = N * M

ans = 0
for i in range(1, N):
    ans += (i * M * M * (N - i)) % mod
    ans %= mod

for i in range(1, M):
    ans += (i * N * N * (M - i)) % mod
    ans %= mod

factmod = FactMod(NM - 2, mod)
ans *= factmod.comb(NM - 2, K - 2)
ans %= mod
print(ans)