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

int expected_number(int a, int b, long long t) {
    return (long long)(a * 0.5 * t + b * 0.5 * t) % (1000000007);
}

int main() {
    int a, b;
    long long t;
    cin >> a >> b >> t;
    cout << expected_number(a, b, t);
    return 0;
}