#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main()
{
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    long long int a;
    long long int b;
    long long int time;
    long long int modulo = (10 ^ 9) + 7;
    cin>>a>>b>>time;
    a = a % modulo;
    b = b % modulo;
    time = time % modulo;
    int res = (int)((a * time * 0.5) + (b * time * 0.5)) % modulo;
    cout<<res<<endl;
    return 0;
}