#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 t{};
    int a{};
    int b{};
    int numCells{};
    
    cin >> a;
    cin >> b;
    cin >> t;
    
    numCells = static_cast<int>((0.5 *a *t) +(0.5 * b * t)) % (10^9 +7);
    
    cout <<numCells;
    
    return 0;
}