import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
    private static final double PROBABILITY = 0.5;
    
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        long t = in.nextLong();
        long results = 0;
        
        if(isValidInt(a) && isValidInt(b) && isValidLong(t)) {
            for(long i = 1; i <= t; i++) {
                long buffer = ((long) ((PROBABILITY * a * i) + (PROBABILITY * b * i)));
                results += buffer % (Math.pow(10, 9) + 7);
            }
        }
        
        System.out.println(results);
    }
    
    private static boolean isValidInt(int i) {
        return i >= 1 && 1 <= 100;
    }
    
    private static boolean isValidLong(long i) {
        return i >= 1 && 1 <= ((long) Math.pow(10, 18));
    }
}