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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
class ChokudaiSpeedRun002 {
    static int __cnt = 0;
#if LOCAL_ENVIRONMENT
    static string __mark = "@@@";
#endif
    static void Main(string[] args) {
#if LOCAL_ENVIRONMENT
        StreamReader __sr = new StreamReader("stdin.txt");
        string __str = __sr.ReadToEnd();
        __sr.Close();
        int __index = __str.IndexOf(__mark, 0);
        while (__index != -1) {
            __index = __str.IndexOf(__mark, __index + __mark.Length);
            __cnt++;
        }
        if (__cnt > 0) {
            Console.Write("Eenter test data no(1-{0})? ", __cnt);
            Console.Out.Flush();
            if (!int.TryParse(Console.ReadLine(), out __cnt)) __cnt = 1;
            Console.WriteLine("Select/Default test data: {0}", __cnt);
        }
        var stdin = new System.IO.StreamReader("stdin.txt");
        Console.SetIn(stdin);
#endif
        SimpleScanner sc = new SimpleScanner(__cnt);
        long N = sc.NextLong(), M = sc.NextLong(), K = sc.NextLong();
        Func<long, long, long, long> MPow = (x, n, m) => {
            long ans = 1;
            while (n > 0) {
                if (n % 2 == 1) {
                    ans = ans * x % m;
                    n -= 1;
                }
                x = x * x % m;
                n /= 2;
            }
            return ans;
        };
        long mod = 1000000007;
        long a1 = 1;
        for (var i = 0; i < K - 2; i++) {
            a1 = a1 * (N * M - 2 - i) % mod;
            a1 = a1 * MPow(i + 1, mod - 2, mod) % mod;
        }
        long ansX = 0;
        for (var i = 1; i <= N; i++) {
            ansX += a1 * i * (N - i) * M * M % mod;
        }
        long ansY = 0;
        for (var i = 1; i <= M; i++) {
            ansY += a1 * i * (M - i) * N * N % mod;
        }
        Console.WriteLine((ansX + ansY) % mod);
    } //Main

    public static int LowerBound<T>(T[] array, int start, int end, T value, IComparer<T> comparer) {
        int low = start, high = end, mid;
        while (low < high) {
            mid = ((high - low) >> 1) + low;
            if (comparer.Compare(array[mid], value) < 0)
                low = mid + 1;
            else
                high = mid;
        }
#if LOCAL_ENVIRONMENT
        Console.Error.WriteLine($"LowerBound(array:{array.ToStringA()}");
        Console.Error.WriteLine($"LowerBound(start:{start}, end:{end}, value:{value.ToString()}) return:{low}");
#endif
        return low;
    }
    public static int LowerBound<T>(T[] array, T value) where T : IComparable {
        return LowerBound(array, 0, array.Length, value, Comparer<T>.Default);
    }

    public class BIT {
        int n;
        int[] data;
        public BIT(int size){
            n = size + 1;
            data = new int[n];
        }
        public void add(int i, int x) {
            while (i < n) {
                data[i] += x;
                i += i & -i;
            }
        }
        public int sum(int i) {
            int ret = 0;
            while (i > 0) {
                ret += data[i];
                i -= i & -i;
            }
            return ret;
        }
    }

    public class Item : IEquatable<Item>, IComparable<Item> {
        public long value { get; set; }
        public long count { get; set; }
        public Item(long count, long value) {
            this.value = value;
            this.count = count;
        }
        public override string ToString() {
            return $"[value:{this.value}, count:{this.count}]";
        }
        public bool Equals(Item other) {
            if (other == null) return false;
            return this.value == other.value && this.count == other.count;
        }
        public override bool Equals(Object obj) {
            if (obj == null) return false;
            Item item = obj as Item;
            if (item == null)  return false;
            else return Equals(item);
        }
        public override int GetHashCode() {
            long hc = this.value * this.count;
            return hc.GetHashCode();
        }
        public int CompareTo(Item other) {
            if (other == null) return -1;
            if (this.value == other.value)
                return this.count.CompareTo(other.count);
            else
                return -1 * this.value.CompareTo(other.value);
        }
        public int Compare(Item x, Item y) {
            if (x == null) return 1;
            else return x.CompareTo(y);

        }
    }

    public class SimpleScanner {
        private int cnt;
        private string[] items;
        public SimpleScanner(int cnt = 0) {
            this.cnt = 0;
            var sb = new StringBuilder();
            var s = Console.ReadLine();
            while (s != null) {
                sb.Append(s + " ");
                s = Console.ReadLine();
            }
            this.items = sb.ToString().Split(' ');
#if LOCAL_ENVIRONMENT
            if (cnt > 0) {
                int index = Array.IndexOf(this.items, __mark + cnt.ToString());
                this.cnt = index + 1;
            }
            Console.Error.WriteLine("SimpleScanner.cnt: {0}", this.cnt);
            Console.Error.WriteLine("SimpleScanner.items: {0}", this.items.ToStringA());
#endif
        }
        public void SetCnt(int n) { this.cnt = n; }
        public int NextInt() {
            return int.Parse(this.items[this.cnt++]);
        }
        public long NextLong() {
            return long.Parse(this.items[this.cnt++]);
        }
        public string NextString() {
            return this.items[this.cnt++];
        }
        public int[] NextIntArray(int n) {
            if (n < 1) throw new IndexOutOfRangeException("引数は1以上であること！");
            var result = new int[n];
            for (var i = 0; i < n; i++) result[i] = this.NextInt();
#if LOCAL_ENVIRONMENT
            Console.Error.WriteLine("{0}", result.ToStringA());
#endif
            return result;
        }
        public long[] NextLongArray(int n) {
            if (n < 1) throw new IndexOutOfRangeException("引数は1以上であること！");
            var result = new long[n];
            for (var i = 0; i < n; i++) result[i] = this.NextLong();
#if LOCAL_ENVIRONMENT
            Console.Error.WriteLine("{0}", result.ToStringA());
#endif
            return result;
        }
        public string[] NextStringArray(int n) {
            if (n < 1) throw new IndexOutOfRangeException("引数は1以上であること！");
            var result = new string[n];
            for (var i = 0; i < n; i++) result[i] = this.NextString();
#if LOCAL_ENVIRONMENT
            Console.Error.WriteLine("{0}", result.ToStringA());
#endif
            return result;
        }
        public int[][] NextInt2Col(int n) {
            const int M = 2;
            if (n < 1) throw new IndexOutOfRangeException("引数は1以上であること！");
            int[][] result = new int[M][] {
                new int[n],
                new int[n],
            };
            for (var i = 0; i < n; i++) {
                result[0][i] = this.NextInt();
                result[1][i] = this.NextInt();
            }
#if LOCAL_ENVIRONMENT
            Console.Error.WriteLine("NextInt2Col: [");
            for (var i = 0; i < M; i++)
                Console.Error.WriteLine("  {0},", result[i].ToStringA());
            Console.Error.WriteLine("]");
#endif
            return result;
        }
    }

}
static class Extensions {
    public static string ToStringA<T>(this T[] array, string separator = ", ", string braces = "[]") {
        string bL = braces.Length > 0 ? braces[0].ToString() : "";
        string bR = braces.Length > 1 ? braces[1].ToString() : "";
        return bL + String.Join(separator, array) + bR;
    }
}