#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 */ 
    int a, b, t;
    cin >> a >> b >> t;
    long double result=(int)((0.5*t)*(a+b))%(1000000000+7);
    cout << result;
    return 0;
}