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

public class Solution {

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

        int a = sc.nextInt();
        int b = sc.nextInt();
        BigDecimal time = sc.nextBigDecimal();

        BigDecimal result = time.multiply(new BigDecimal(a)).divide(new BigDecimal(2));
        result = result.add(time.multiply(new BigDecimal(b)).divide(new BigDecimal(2)));

        result = result.remainder(new BigDecimal(1000000000).add(new BigDecimal(7)));

        System.out.println(result);
    }
}