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

const long long mod = 1e9 + 7;

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    long long t, a, b;
    cin >> a >> b >> t;
    cout << t * (a + b) / 2 % mod << endl;
    return 0;
}