import java.util.Scanner;

public class Barrels {

    public static int A;
    public static int B;
    public static int K;
    public static int C;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        while (sc.hasNextInt()) {
            A = sc.nextInt();
            B = sc.nextInt();

            K = sc.nextInt();
            C = sc.nextInt();

            int ll = K * K;
            
            if (C == A || C == B) {
                System.out.println((ll / 2 * K) % 1000000007);
            } else {
                System.out.println(0);
            }

        }
    }

}