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

#define LIM 1000000007;

int main()
{
    int a, b;
    unsigned long long t;
    cin >> a >> b >> t;
    
    unsigned long long value = (a+b)/2;
    t %= LIM;
    unsigned long long output = (value * t) % LIM;
    
    cout << output << "\n";
    
    return 0;
}